Hi
There is latex hardener. You mix it with latex and then let it dwell in the mold as you would with plain laex, you can play with different ratios depending on the hardness you need. Take in account the lockings your sculpt might have as the casting will be more rigid and less flexible and stretchy (and your mold is rigid plaster). You should be able to find it where you buy your latex.
There´s also a product called "neoprene latex". It´s not the neoprene for scuba diiving suits. I Think it´s what they call "synhetic latex", never used it but it should be stiffer.
Also, you can use fiberglass with latex. I´ve used this method many times for helmets, armours and other light weight props, specially for theatre. But it will be important you protect the interior with some other material (I like to use EVA foam) to prevent any chance that the skin touches the fiberglass. Along time it might start coming out, so better protecting it with something.
You can get light weigh/rigid castings with this.
If you try this route, you should brush three layers of latex (better than dweling) and then lay fiberglass. If it´s a complete mask you might probably have to do this with the two mold parts separate and the close them together with more latex+fibegalss. Too much latex in the previous coats or too many fibreglass coats can result in a "carpet like" and heavy material. One 300 matt fiberglass coat will be more than enough, you can also try with 200 depending on what you are after.
This method could give some wrinkles when demolding. I´ve sometimes solved this by adding latex hardener to the first layers, but i doesn´t always work perfect (as for the wrinkles).
One downside with latex hardener is that it decreases the durability of the casting. I have some castings sitting around in the studio that have become cracky and too rigid along the years, faster than plain latex ones.
