Nothing wrong with that case Bill, it's the sort of thing I'm thinking of.
I have a friend who has a contact at a display and sign company, who has offered to cut the acrylic/Perspex if I end up using that instead of the thinner stuff. Then it's just a case of either bonding the butt-jointed edges, or setting them into the 90 degree wood mouldings.
One way of obtaining the correct acrylic sheet, rather than having to purchase fairly expensive large sheets, is to use the fronts from those clear 'clip frames' used for framing posters. One, maybe two of the largest size frame will provide enough raw material to make a 'box' up to about 12 x 18 inches, perhaps slightly larger, depending on the height required. These can sometimes be found, in the largest size, for as little as £10 each, with average price being nearer £15 to £18. Still not cheap, but a darned site less than the average acrylic sheet price, and worth it, to preserve a model which has had a lot of time and effort invested in it, plus the cost of the kit and any accessories.