Rick & David,
I have some perma-nest trays that have the humidity grids that can also be used to create a reservoir when using capillary matting. These:
http://growerssolution.com/PRO...
I can't swear to it but I am almost certain that they are just eggcrate light diffusers that have been cut to size. Like this:
http://www.homedepot.com/p/Lit...
From one panel of light diffuser you should be able to make 4 grid inserts for a 1020 tray. You could stack more than one for a deeper reservoir. Unlike a block of solid material, the grid not only provides a stable surface for the pots but also allows more area for water to be stored and, therefore, increases the volume of your reservoir.
I'm sure you could also find some small items to use as spacers under the light diffuser grid if you didn't want to use multiple grids in one tray to increase the reservoir. Perhaps cutting evenly sized rings from a shampoo bottle or similar would work for spacers on the cheap. A food/water safe type of plastic pipe cut in even rings might also work.
