For $16 you can't go too far wrong!!
Those firewall gaskets are just cheap foam. I see guys selling them at swap meets for $5 or so, but if you have some Styrofoam sheets used as packing for shipping, I think you'd be fine using it to cut and trim a piece you can use instead.
For my PB cars, I haven't even bothered to use a gasket between my MC and booster the last few times I changed master cylinders, and it doesn't seem to matter.