Building the Base
Cut a piece of plywood that is 30-15/16" x 20-3/4". You will also need two plywood strips that are 30-15/16" x 4". Take one strip and place it on the plywood base, aligning it 3/8" from the edge (see B). Pop a couple 1-1/4" screws (I actually countersunk in 1" screws instead) to hold it in place. Repeat with the other side. See pictures C and D. Flip it over and test fit it into the side slot. It should fit nicely (as in E). Now set this base safely out of the way. You have more work to do before you need this piece again.