Yep, insulation foam, aka A-B foam works great!, but it could crack your stock if you dont tap enough holes.... (i would personally wrap the stock in Ducttape or something before injecting the stock inside to hold it in place...)
And if it cracks... you can always JBweld the cracks + sanding + paint!
PS: once the A-B foam is cured, it doesnt stick at all! and can be cut/carved with a knife... (just like the pink polystyrene foam)