30" cube terrarium

my latest idea for the 30" cube terrarium is to stack two plastic tupherware containers on top of each other to make the land area. i could then coat it with expandable foam. but i'd like a few inches of space between the land area and the walls of the terrarium which will be filled with water to make it look kinda like an island. the expandable foam will be from 2"-4.5" thick when set. there should be about 4"-5" of space between the walls of the terrarium and the 'island'. the tupherware containers should be about 17" x 17"; possibly a little longer than wide.