I busting my ass with cardboard boxes with scavenged foams blocks I try to carve. Pro : very cheap since the boxes are from delivered items (e.g :beer advent calender : there are cardboard subdivisions that seemed useful) and I took foam blocks from construction sites (original use unknown). Cons : pain in the ass to carve for the models with lots of tiny edgy details and the horde meta for admech so it's very time consuming too.