The most widely spread collagen material that could do what you want is
Gelatin + glycerin
This is also known as ballistic gel and often used to make molds for chocolate or fondant. For a hard, rubbery consistency mix 2 parts glycerin and 1 part water (by volume), dissolve 2 parts gelatin in it and heat the mixture to melt it. You might want to add an fungizid to avoid mould. Don't boil it.
Alternative recipe for 7,5 liters of a long-lasting ballistic gel by the German channel Hunting-Gear
- 1215 g ground gelatin
- 3750 ml destilled water
- 1875 ml glycerin
- 1875 ml isopropyl alcohol
If you want a softer consistency, add more liquid in the given ratio. The finished mixture can be melted again and again. Gelatin has a yellowish-brown color and is translucent, but not transparent. There are some special products that promise a glass-clear gelatin that are worth trying out.
According to several sources this gel can last more than a year when stored in the fridge. If it's sealed in a container the alcohol should be sealed in as well, hindering the growth of mould and bacteria.
Video tutorial on YouTube by Lovecraftforever
Chemical explanation and a slightly more complicated, weight-based recipe by observationsblog.com
Agar-Agar also forms a gel, but it has a consistency that I would describe as "brittle". The worst property of agar gel (for your application) is that microorganisms really like to grow on this stuff. That's why it's used in laboratories as culture medium. I would rule it out for a long-term project.
In "molecular kitchen" settings we also find sodium alginate, which forms a gel shell around a liquid. I'm not sure you could even create a big, uniform block of sodium alginate gel, though.
Iota Carrageenan and Kappa Carrageenan are usually used in the kitchen to gel dairy products. Iota Carrageenan forms elastic gels like gelatine, but Kappa Carrageenan forms a brittle gel like Agar-Agar.
Methylcellulose only forms a gel when heated and returns to a liquid state when it cools down.
There are collagens of diverse sources that are usually only used in laboratory settings. The technical term is "collagen hydrogel" and it seems to be a field of study on it's own. I'm not sure how applicable these materials are for your scenario, but you'll get a ton of results when searching for it.
An interesting list of food-grade gels can be found in the Amazing Food Made Easy blog.