Found a drawer table thing in the garage and decided I might as well build a rack. I think I have it planned out. It is going to have 3 shelves, the bottom 2 will have 1 sterilite 41qt and the top will hold 2 sterlite 35qt. I was wondering about how much space should I leave between the top of the tubs and the shelves without having lids (don't want any escapes). Also which size flexwatt should I use for belly heat? Can the tubs sit right on top of the flexwatt or should they be raised? Any other suggestions feel free to throw them this way. Thank You