don those backgrounds can be constructed in many ways. some may use that expanding foam, others will use a flat sheet of styrofoam and then apply several coats of cement w/ colors to get the effect they want. being this is a company and not some regular person i would say it's constructed of some heavy duty plastic or other reef safe material.
how do they get the acrylic or glass mounted as the window panel? i was always trying to figgure out whats the biggest and savest size wood and acrylic tank?