For short run stuff, or an intermediate mold I wouldn't form directly over more than a few times, I might use something like water putty or gypsum cement, with drywall tape reinforcement.
For something really durable, I'd probably go with a resin impression coat, and lay up some fiberglass of some sort under that. Maybe just a healthy layer of Bondo-Glass. (Which is not like regular Bondo. It's not easy to sand, and it's much stronger, because it's filled with glass fibers instead of little microbubbles.)
If I still wanted to be able to sand and modify it easily, at the expense of making it somewhat less durable, I might start with a layer of regular Bondo. Then, deeper than I'd ever want to sand away, a healthy layer of Bondo-Glass for overall strength.
BTW, don't trust me on this. This resin/glass stuff is all still on my to-do list, not something I've actually done. (Today I'm experimenting with hot wire cutters for the foam part... I'll get there eventually.) I have done plaster waste molds and reinforced water putty casts, but not fiberglass.
Paul