Crafting a sturdy, not heavy mask for shows
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.