When I had a friend make me some, we bought melamine from B&Q for the floor, top and two sides. We then bought a sheet of thinner "crazy paving" patterned kitchen flooring for the back. All of this is water resistant and won't need varnishing, but damp can creep into the joints, so if you want the thing to last as long as possible you can seal joints with an aquarium sealant.
I don't know what sort of glass he used for the sliding doors, but that was cut to size by a glazier.