Thanks everyone - some great tips, I'm happy now to have a go without
the worry that I'm on a fools errand. I think I'll try layering some
Plywood/OSB and creating a raised edge around so there is enough room
to place a sheet of 1/4" Masonite in the 'tray' created. With some
doble sided tape to hold it in place I think it will suit my purposes
well. Thanks again.