biggest (reef safe) container that will fit under a 20long,29,37 stand?

so i bought a reef octopus classic 152 and put it under my 37 tall tank with a 10gal sump, only problem now is that i literally have no room left for my chaeto to grow. whats the biggest tank that i could realistically squeeze under the tank stand? its a 30" long stand by 13" wide but obv the inside isnt as big because those measurements are from the outside of stand.

anyone ever have this stand before? its an aga black pine standard like you would see at a lfs. i want to make a new sump that would allow me to have a small space to grow chaeto.