You're not going to want it too thick and soft as it will get squishy underfoot and it will get tiring trying to stand still... unless you're looking for a constant core workout! Instead, build a simple platform that is wide and stable and the height you prefer. You can build it so it custom fits into the area so it doesn't move. Then, just add one of those interlocking foam floor tiles that you can buy just about anywhere. Depending on the final height, it could be a box with a lid so you have some storage.