I use a few bags of #4 bird shot in some scrap mattress ticking and some lead diving wieghts with vinyl covers to keep the lead from rubbing off. I like the bird shot because it will mold to the shape you need

I just did these panels for a 54 Studebaker on panel board

I use rebond (like carpet underlay only thicker and skim the top with 1/2 inch of high density firm open cell foam. Hope this helps