OK I've not got very far yet, but figured I'd do a "step by step" post and track what I've done and how much it has cost etc :P Measurements are in mm.
I'm now the proud owner of 6 bits of wood! I started off with a 2440 x 610 x 15 bit of white conti board (?13.68), which nice Mr B&Q man cut into a 610 x 610 piece (the back), 610 x 500 (base) and 2 610 x 485 bits (sides). Then a short strip, 610 x 50 to put on the base at the front, and an off cut. The bit at the bottom is to keep the cost of the perspex "doors" down - when I wanted them full height it all got a bit too expensive (cheapest quote for two bits was ?20!), any taller and I thought scraping out the soiled substrate would be a pain.
This is the plan I'm working to...the slightly silly height is due to only being able to find 18mm high sawn timber for the frame, and wanting to get the full 610mm "usable" internal height.