You can improve on PVA glue with what's referred to as "cold porcelain". There's endless information and tutorials online, but it's basically a mix of PVA glue and gelatinized starch, plus a tiny amount of a conditioner, like mineral oil or glycerin. It can be enhanced into an even tougher, more plastic-like material by adding a little stearic acid (used in manufacturing many cosmetics and personal care products, and available to hobbyists for candle-making, so it could be a household ingredient).
Cold porcelain has a consistency more like dough, so it's typically used like modelling clay. But you can press it into a mold and get fine detail. It shrinks a lot, so if matching the original size is important, you can do it in layers. Line the mold with a thin layer, let it dry most of the way, add another layer, repeat. The shrinkage will be mostly in the thickness.
Depending on the additives, it dries translucent or white, rather than clear, or you can add food coloring to it to make a colored plastic. You can reduce the shrinkage by mixing in a filler. However, that makes it much stiffer. You can also start with less water in the mix for the fill layers, which will reduce shrinkage. Another trick is to start with less water, more starch, and not gelatinize all of the starch. Once it's in place in the mold, you zap it in the microwave to gelatinize the remaining starch and it sets up in place as a solid.
You can mold with actual recycled plastic rather than two-part resin. PETE, which is the plastic used for most water, soda and juice bottles, is a good one because it melts into a liquid that flows well into the nooks and crannies of an open mold (which should be made of silicone rubber to tolerate the temperature). You can verify the type of plastic from the recycling symbol if you aren't sure.
There are lots of online tutorials on the details. Here's one on using bottles: How to Melt Plastic Bottles for Molding. Most plastic you recycle won't be a pure resin, it will have additives to give it particular properties. That will affect the melting temperature, and your oven temperature may be inaccurate. So a certain amount of process adjustment is required.
The approach described in that link is to melt the plastic and pour it into the mold. Another approach is to melt the pieces right in the mold. Over-fill the mold with the plastic chips and heat it in your oven. As the plastic melts, the liquid will take up less volume than the solid pieces, so you can add more plastic.
If you're careful to build the temperature and then keep it at the minimum required to melt the plastic, there won't be much in the way of fumes. But if you try to rush it with a higher temperature, some of the plastic will start to break down and you'll get some nasty smells. It's generally best to use good ventilation.
You can also cast polystyrene at room temperature using household ingredients. There are lots of online tutorials for dissolving Styrofoam, but the following approach works best for good quality casting, and doesn't involve toxic fumes.
Collect clean Styrofoam. White foam, like from package inserts, and styro cups and trays, will make almost clear polystyrene. You can make colored plastic by using colored Styrofoam. Meat is often packaged on black styro trays. Egg cartons often use colored Styrofoam (but cut away any parts with printing). Colored Styrofoam will become a solid, opaque color. You can make tinted polystyrene by adding a small amount of colored Styrofoam to clear (white). This is easier to control if you create the resins separately, then mix them and observe the color. You can start with pieces of polystyrene instead of Styrofoam, but that will take a long time to dissolve.
Styrofoam will dissolve in endless solvents, but most will not produce a good casting resin. Among other problems, and toxic fumes and fire hazard, there will be a lot of shrinkage and air bubbles trapped in the finished plastic (apparently from the evaporating solvent, since it happens even if the polystyrene solution is low viscosity and ages until no bubbles are left from the Styrofoam). Limonene, a solvent made from citrus fruit, has all the right characteristics for a good finished product. It's widely available as a general purpose cleaner.
Styrofoam is 95-98% air, so the volume of polystyrene is only 2-5% of the foam you start with. Styrofoam dissolves much slower in limonene than in solvents like acetone, so rather than a spectacular show of reducing Styrofoam to nothing before your eyes, you need a large airtight container, like a plastic paint storage bucket, to produce enough polystyrene to cast.
Fit as much Styrofoam as you can in the container, and pour a few ounces of limonene over it. Seal the container, wait half a day, and check it. Push any chunks of Styrofoam into the liquid. If you will need more polystyrene, add more Styrofoam. If the dissolved Styrofoam is very thick (viscous), or there wasn't enough limonene to reduce the Styrofoam to a puddle of goo, add a little more limonene. Once everything is dissolved and you have a little more than you will need, you can play with the consistency.
You want to start with a thick syrup, like honey or molasses, and you want all of the air from the Styrofoam to have dissipated so there are no bubbles in the liquid. Over the course of a few days, the limonene will diffuse evenly throughout the resin. Some of it will evaporate (even in the sealed container). The resin will become a clear, uniform liquid
As the limonene evaporates, the resin will become a gel. You will be able to pull it away from the side of the container and it won't be sticky. Prior to this point, some of the resin volume will be excess solvent, so there will be shrinkage of that percent when it dries. Once it turns into a gel, there won't be much shrinkage; what shrinkage there is will be mostly in thickness.
Scoop the gel into the mold. It will slowly flow into the nooks and crannies. Wait for it to flow, then add more gel until you completely fill the mold. Give it about a week for all the limonene to evaporate (you'll know it's finished when there's hardly any citrus aroma). Then remove it from the mold. The surface that was exposed to air should be very hard. The surface against the mold should be solid, but may still be a little soft. Set the casting mold-side up and let it dry for a few more days, especially if that side has any citrus aroma.