Sump for perfecto 120g setup

[newbie2]

Hello all,

not sure which forum to post this in but here goes:
I bought a perfecto 120gallon setup (4x2x2) with tank/stand at the local BigA boxing day sale, just waiting for it to arrive. Excited to make the upgrade from my 20gallon setup but I am a little worried about the size of sump that will fit in the stock perfecto stand (believe its a monarch stand but similar to the old perfecto stand with the large center brace).

Was hoping to either use a 40breeder and make my own baffles or get one custom made if I can nail down the dimensions.

Does anyone out there know what size will fit in/out of these stands?

thanks