Also another alternative is to use a piece of cloth (or several pieces) to cover the base or shelf of the display case. Then you can use whatever solid object (rigid cardboard box, light block of wood, various plastics, CD cases, etc) to raise a platform underneath for your figures to stand on. Just make sure the size of the cloth is big enough to cover the base and the object you're using to raise a platform. My wife does a lot of home crafts so following her to the craft store, they sell cheap pieces of cloth of various colors and patterns.
Of course, with a clear riser you'll be able to display smaller figures underneath. I have a few acrylic risers but I still cover them in cloth.