Thanks for the advice. Going to be ditching the Spackle. I actually found this product which looked interesting:
Just add water.
Apparently it can hold up to kids jumping on it and is sandable.
I ordered one and I'm going to be testing it soon. If it turns out rough after sanding, I'll probably coat it in Modge Podge to smooth it out.
Took me like 12 hours to get the general shape of the backpack.
I just need to finish sculpting the back spine protector and a few more details and I'll be coating it in the foam coat.
While I'd love to go with fiberglass or resins or some fancier materials, my experience is lacking and my work area is rather limited unfortunately.