steph, if your friend has a chopper gun, fiberglass is easy to work with.
to make the mold:
use turtle wax and generously wax the inside of your tub.
Then you will mix and brush on a nice even layer of resin/gel coat.
If you have a chopper gun, you would then spray fiberglass over the gel coat.
Then dab resin till the shredded glass looks wet
repeat the chopper gun resin steps until it look stick enough for your likings (atleast 3 times, 6 would be great for a mold)
Let it dry.
next day you will pop your plug.
pop the plug from your tub and trim a clean edge
I would use plywood to make a base/flange
T make actual parts:
this time you want to wax the mold twice as much
lay the resin on thick, and then the first layer of glass. Smoother the resin, the smoother your inside of the tub will be. Spraying it on would be ideal. Then repeat like you did for the mold until you reach the desired thickness.
so an idea on how to fiberglass
go to youtube and type in "how to fiberglass"
there are actually very good videos there on how to.