Hey FEF, I just thought of this while reading through this thread....
To keep from having any rockfalls in your reef, put four stilts to hold up the eggcrate with cut holes of a certain diameter to allow the mantis burrowing sites. Then stack the rock work on the eggcrate where the holes arent fill up with sand and he is only allowed certain openings which you can make it so that his opening is in the front of the glass so you can see his whole body while buried this way he can dig as many tunnels as he wishes but your rock formation is completely safe. (If I confused you just ask me I can go into more detail with you)