Ha! I'm far too embarassed to take a photo of my crude eggcrates shelves, but all I did was cut a piece that would fit where I wanted it, then used Weldon to mount a second piece of eggcrate at a 90 degree angle to make a lip. I then use a Magfloat on the lip to hold the shelf.
In retrospect I should have used acrylic or maybe some plastic light diffuser material because the eggcrate being 1/4"-3/8" is a little too thick for the Magfloat to give a really study hold. Something thinner is definitely called for, for the attachment lip anyhow. You could do a little rim still out of eggcrate just to help keep frags on the shelf since snails loooooooooooove knocking frags over in their little games of snail-frag-soccer.. ("Ooh! Frags! Fun!"
)