Bigred made a tutorial for padding with foam and covering with material. Here's a link
http://www.405th.com/showthread.php/27863-Armor-Padding-Tutorial
As for the rondo issue, if you do a hybrid like I've been doing on some pieces, make sure the rondol mix is very thin with lots of resin! On my current suit I think it was more like a 77% to 30% mix of resin to bondo. My forearm pieces are stil light enough to wear cause the mix is so thin it just coats the inside and fills some of the edges so I can put a layer of glass over it. But like Ventrue said, if the bondo mix is really thick your piece will be pretty heavy. If you have some spare parts layinmg around, try both methods and see what you like best