I cut strips of eggcrate, foamed them, dropped shells in for texture, and glued the eggcrate strips to the glass and plastic overflows with a tube of silicon. Then I filled in the gaps between the glued up strips with more foam straight from the can.
The finished tank looks great (one day I'll do a build thread...) The only thing I might do differently is to use a darker foam.
One hint: when you fill in the gaps after the strips are glued up, its hard to control the amount of foam, and it expands between the seams, so you get a raised seam. Go slow, put in less than you think you need. If it expands too much, after about twenty minutes it forms enough of a skin that you can pat it down with your hand and work it flush with the strips.
Suggestion: practice on some scrap first. If I remember, I was able to use the can over a twelve hour period without it totally clogging up, although I tried to lay out enough work that I would use an entire can in one session.
Good luck!