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.