I cut 4 23" square pieces of plywood from a 4x8 sheet 1/2 inch thick
2 pieces 22.5"
I made two frames from 1x6 pine. Cut down the center to make them 3 inches wide then cut 4 pieces 21" and 4 at 22.5". So I used 2 21" pieces and 2 22.5" pieces glued and screwed together to make the frame. Then I fastened the square pieces to the frame. This made my box with an inside of 27". Then I lined it with 1" thick industrial insulation, the hard stuff not the soft stuff. I doubled it up so it's 2" thick.
I'll post more pics when I get it all done.
Oh and the clip sounds good so far. everybody's x are the right angles removed on the inside of your cabinet?