I have seen people on the forum with pics of a grid-like material at the bottom of their humidity bins. I've been using a cookie rack which works fairly well, but it doesn't always sit nicely and isn't exactly the dimensions of my bin. I went into a hardware store and struck out. On ebay and google I had no idea what to search for, looking for "grid," "grate," "plastic drain," "drain grate," etc. Finally, I found the magic phrase that gave me a lot of results that looked like the grid I wanted: "eggcrate."
On homedepot.com (it is also on lowe's), I searched for "eggcrate" and found this link: http://www.homedepot.com/p/Plaskolite-4-ft-x-2-ft-Suspended-Light-Ceiling-Panel-1199233A/202025149
Interestingly, this is a lighting panel shelf, but it seems to do the trick for fairly cheap. It could stand to be a bit thicker, but at .375 inches it should work ok.
I was able to cut 3 full size sheets for the bottom of a sterilite bin (about 19" x 13"). The remaining parts were enough for a 4th bin, albeit in two pieces. In the image, the blue represent the full size pieces and the red represents the two-part pieces. The plastic is very brittle and tends to shatter as you cut through it with heavy duty shears (watch the eyes). Even scissors would probably do it.
I suspect that this lighting panel is not what some others have used, but it was all I could find other than some overpriced fiberglass grating ($200-$300). If you have a better source, feel free to chime in. Hopefully this saves someone else some time looking around!
