So recently I tried vacuum forming my pep/fiberglass/bondo visor buck on my homemade vacuum form. It worked very well, except one big problem: The shell itself collapsed under the vacuum pressure. After posting to the Facebook page of my division (Southern) I realized that backfilling props like that is very important. However, I am now curious about what material to use for backfilling on pep'd pieces to make them strong enough for forming. Any help is greatly appreciated! Thanks!