Spend the princely sum of £1 at a poundshop on a picture frame, remove the glass & there's your base! Or find your nearest timber merchants, they'll have some MDF off-cuts, give them a fives to trim & bevel the edges. Then just varnish it to stop it warping. Patrick
Not done a diorama yet , still in its planning stage but I`m going to use 1/2 inch MDF , any B&Q will do it and its cheep , even cheeper if you ask for the off cuts , once primed anything will stick to it,