I'll pass on a tip I use, learned from some '80s-era modeling magazine...might have been the old Scale Modeler or one of the IPMS/USA publications.
Use the thin styrofoam from foam-type egg cartons. All you have to do is rough-cut a blank of the approximate size...slightly larger is ideal...then just pop it in and glue with PVA or some other non-solvent adhesive. The foam neatly compresses to perfectly fit the contour with virtually no bulging or mis-shaping (if that's a word). Best to paint with acrylics; I occasionally tried the old Testors 'square bottle' enamels (my go-to during those years), which usually had no effect...but sometimes would start to corrode the material over time.
BTW, if you're of a curious nature, and have never tried it, 'test' any solvent adhesive or regular tube glue on egg-carton foam: it eats it away before your very eyes, just like acid. Definitely not recommended for modeling work. 