I think my two biggest mistakes came from never having worked with the foam. I think the fabric probably got tight because I really rushed filling it with the foam. I didn't realize how much time I had to work with the foam. I think that my rushing to get it done as fast as possible made me perhaps put the board on too soon without allowing for extra fabric in the corners. Next time I make it I'm going to take my time and really place the foam where I want it and make sure that it stays in that area when I pat it down. I will also be using a black t-shirt
and way more than 20lbs of weight when I do set the boards on.
Also, I used the t-shirt as a double layer, so once the foam was all cured and I was cutting off the excess I realized that none of the foam was touching the second layer. At that point I just pulled the second layer off all together. Maybe that left it with the appearance of being much tighter?