Hey niloc16 i just made two of those a couple months ago for my gsp and ackward shaped frags, it works great, made it so my green star polyps don't invade everything around them!
As you mentioned the best and most stable way to go about it is 1/2'' eggcrate and zapstraps for the sides, and just push it into you're sandbed. Another great thing is that if you use the 1/2'' eggcrate, it works with the coral cradles. btw, Wicked Frags!!!
can't wait till I fill mine up like that