We built Montessori curvy plywood balance boards. They are 36" x 11" deep by 3/4" thick. They are made of 3 layers of maple veneer plywood. I'm unsure of the exact classification of the plywood as I purchased it a few months ago.
These balance boards are glued together and bent over a form. We have tested them with adults around 180lbs with no issue. But parents would like to know the weight capacity before purchasing and I'm unsure of how to get an accurate estimation.