My brother uses a product called fibertech. It looks like the "kitty hair" which is the fiberglass reinforced filler. Fibertech still uses a herdener like bondo, but this stuff made to bond to different types of plastic. It can be found at autobody supply stores.
Check out MH Ready Patch by Zinnser. It's at an alkyd based spackle available at Home Depot. I have yet to use it on a plastic or F/G prop, but it works great on wood and drywall. No mixing, dries hard, and only kind of stinky.
Depends on what youre filling. Are you filling spots on a helmet or large gaps in something? Bondo is essentially made to stick to metal and fiberglass, its a automotive products after all.
Here are some alternatives:
PC-11 Marine Epoxy
PC-7 Epoxy (both available in the glue/epoxy section of Home Depot)
Squadron Putty - white hobby putty/paste, dries hard
Apoxie Sculpt - 2 part sculpting putty, can be sanded and sculpted
Zap Finish Resin
All great products, you can google them, find them at pro-level - R/C hobby shops, etc.
I use them all, for different needs. Heck used them all on a clone armor build, all is seamless, most of them I smooth out with water after putting them on, less sanding and better "seamless" results.