Hi

MDF has a bad tendency to sag over time, Just place a scrap length say a metre long between two supports and leave it for a few days it will sag by its own weight and stay bent. If you do use it use plenty of supports under the sheet

Concrete form ply is worth looking at. (Not the cheap home hardware store kind) I went to an engineering exhibition recently and found a product called Doka Xlife Plywood the sample I have is 20mm thick and is made up of about 15 ply's including a plastic wear surface on one side.

I have it on my list as a Router bed material. I plan to drill the bed (With the CNC router) with a 50mm spaced hole pattern for threaded M8 metal inserts, available from cabinet hardware suppliers. This ply is extremely dense. I am sure other competing form ply suppliers will have similar product.

I have no connection at all to the company.

Regards
John