I recently used a A2 sheet of 5mm foam card to build some stepped contoured hills. I sketched out a design that left a minimum of 30mm between each step, and cut the shapes out with a hobby knife. Then I went back about the edges of each shape with the hobby knife to roughly cut out a 45 degree slop...

I use that plastic coated corrigated card used in packaging. I cut out the bases with a hobby knife and ruler, attach the figures on with wood glue, and apply fine surface polyfilla to hide the edges. It's free. It doesn't warp. And it's easy to rebase figures should I choose to. I did worry when I ...