personally, if it were me, i'd simpldo some careful trimming with scissors on a piece of cardboard. take your time and make it fit like a glove. Then, when your done, grab some particle board or something else inexpensive and use a jigsaw to cut it to shape. Then, grab some little metal elbows or something from the harware aisle and simply screw it in the top of the board, it should act as a clip to hold the middle in place with the cent of the tub...it wouldnt be too terrible..

as for the smaller tubs, follow the exact same procedure, simply instead of using the 32qt tubs when creating the bottom levels, use the 6qt tubs. it will end up leaving you a little extra room on the top level, but thats no big deal as thats just extra storage anyway.