Well, magnets come to mind first. It will take some rigging and a few tools, but if you cut the pvc to that it cradles the edge of the tank and the edge of the eggcrate, that might work. Or you could just use superglue gel and put a few dabs between the eggcrate and glass. That's not permanent and can be scraped off later. That's really all I can think of right this minute.
