I worked out the spacing on the 1x4 s that runs along the short side so that the groves on the bottom of the rubbermaid would be set inside, but the flexwatt was not really close to the bottom so I added an extra piece of 1/4 in. plywood to get it up to the bottom.
The rack itself cost me about $30. I got the tubs from target for about $8.00 each. And all my heating stuff came from matt and cost about $150