Thanks NT, your square inside a square is a great idea, I was thinking one diagonal from bottom left (hinge side) to top right like a gate.

and thanks Lee, I was thinking 6mm MDF only because I have a large piece left from a previous project but you are right 3mm hardboard would be better.

One of the reasons I was thinking about the solid MDF was because of problems getting wood that is the right size and also straight. I confirmed this problem in B&Q today, ideally I would want something about 30mm thick but it's all either 22 or 34 and all as bent as a banana. I will check the local timber yard tomorrow.