You really do need a design wall. I tried working on the top without it, but gave up. I found a couple of sheets of that pink insulation foam board and covered it with cheap felt. However, when my dog, Buddy, walked by, he kept rubbing the fabric blocks off. So, I went to just stabbing straight pins into the corners of the blocks to hold them on. It works, but not perfectly. And it is a little more of a hassle than just having them stick to the felt.
Once I could see what was happening, it was much easier.