There's been quite a few different products that folks around this forum have tried with varying degrees of success...
Note that I do not use the phrase "success and/or failure"- even "failures" in the spirit of this forum are in-themselves limited successes as we'll at least know what NOT to use, right?
In the spirit of the forum as well- the types of questions such as this are supported HEAVILY within the forum- the old adage "The Search Function Is Your Friend" would still apply outside of this Noob area- so the products I bring up, make sure to follow up with a forum search- you're going to be amazed what's in here!
Products I've seen include
Aqua Resin- which is a resin but it doesn't have the same potential chemical and fume problems as polyester/fiberglass resins [you'd still need at least a Level 2 dust mask as the powdered resin can become airborne incredibly easy!]
Hot glue- as already mentioned- though it has an odd effect here in Florida where I'm from- if set in the sun it can get tacky after a while!
Some have mentioned
Paper Mache- which, if you add white glue to your mix may be helpful [again- check the Search function on this one- there's debate as to "Halloween Grade" versus a form of "Screen Original" grade]
There's an "old school" product out there that I've not seen mentioned called Cabosil- which is a cross between Paper Mache and Fiberglass- the problem is you'll need to know how to use BUCKETS of ACETONE to heat the stuff up and you'd have to do it outdoors ALSO wearing a gas-mask as this stuff is
incredibly dangerous fume-wise... I only mention it because I used to work with it back in the 1980s and even then it wasn't terribly popular other than it was a fast-working product.
One that I myself am highly interested in is a spray-on truck bed liner that's sold in auto departments of Walmart and places like Auto Zone... Word has it that it hardens like a hard plastic helmet ought to! If indeed that's the case and it'll hold Bondo well, I may go that route just for expediency!
I know there's probably more that I'm missing, but perhaps that'll be enough to get your own creative juices flowing and you'll be able to come up with some of your own ideas!
Just don't forget to share- right?
RusSEAL Out.