I have a metal band that use costumes for shows, I need to craft a mask that is durable but not too heavy, since I will use it on live presentations. I tried to make it using plastered gauze, but it seems too flexible.
Plaster that is thick enough to be reasonably strong and rigid will probably be too heavy.
Masks are often made from papier mache, essentially building up strips of paper soaked in paste over a former which dried to a fairly strong but lightweight material. Other methods involve pressing paper pulp into a mould.
You could also use your plaster original as a former as long as you coat it with wax, Vaseline ect to act as a release agent and prevent the paper from sticking.
An quick internet search will find many tutorials on making masks in this way.
The big advantage of papier-mache is that you don't really need much in the way of tools and equipment to work with it and teh raw materials are cheap and easy to get hold of.
There are also many other options depending on your particular priorities, moulded leather, sheet metal or cast resin or composites would all potentially be pretty cool but there you are looking at either a substantial investment in time, practice and materials or engaging a specialist to work with you to make it.
Having said that you may find that if you reach out a bit you may find an artist of fabricator who is sufficiently interred in the project to give you a good deal. I would certainly be interested in something like this but equally if you want at a good deal based on collaboration you need to be prepared to give up some creative control of the project. So you need to make sure that you have a similar vision in mind.
While papier-mache is undoubtedly the easiest way to make this type of 'plague doctor masks', the originals were made of leather and if you have the skills/tools/patience to do so you may find that you prefer the look (if not the weight) of the results.
My go-to materials for something like this would be craft foam or Worbla. These two materials are easily heat-formed into lightweight, reasonably sturdy pieces. For craft foam, a gas stove or heavy duty hair-dryer can be used as the source of heat, but for Worbla something more substantial like a heat gun is ideal.
I'd recommend first making a pattern for the mask out of taped-together construction paper and then using that to cut out & shape foam or Worbla pieces to your liking.
If I didn't have the fifty bucks to buy one from a costume supply and had the time and patience to create; my affordable project would need:
old adjustable ball cap
Velcro or leather boot laces (laces are more attractive)
Hot glue or stapler
cardboard (not corrugated, unless in a pinch)
metal stud from Thrift Store or Dry wall nails
Pleather, naugahide, or any strip of faux leather
Now cut the bill of the ball cap to form a beak. Then remove the crown as well, but leave room for the eye holes. Should the adjustment strap on the back not be enough, add the Velcro or (my preference) leather laces. Make room for the bridge of your nose if need be, then try on the mask foundation. Make it comfortable and then proceed.
Take your cardboard (old monopoly box top), cut out a long slim triangle and bend to shape. If you wet it the bending will be easier and the board will hold some of the form when dry.
Now staple or glue the formed beak to the bill of your cap. When affixed move to the faux leather. Cover the beak and, if there is enough, make a hood. Glue or staple to cap. Trim off excess. Don't forget the eye holes.
Find some Krylon and spray every bit of the creation in your favorite color or lack of color in this case. Then stud it up, hiding the staples. Drywall nails have large heads and bend easily (aluminum) and can be as eye appealing as metal studs. They will need glue to stay in place.
Now go into the world protected from biting fleas and good luck at your future events.
If you can borrow or build a vacuum former it's quite easy, especially as you have something to use as a mould (which might need reinforcing but you would no longer care about the weight). Many plastics will vacuum form; some of them are easy to cut out of the parent sheet once cooled and some of themwill take appropriate paint very well.