MDF or ply?

[EddyCurrent]

John,
Funny how you overlook things you've used before, the M8 inserts is a great idea, I'd recommend these too, Toolstation > Screws & Fixings > Nuts & Washers > Pronged Tee Nut
It's true that MDF is iffy stuff and I normally avoid because of dust and Urea-formaldehyde release but for a sub-bed that is fully supported it's relatively cheap and flat.
For now I'm sticking with Birch Ply for the main bed + M8 pronged nuts and I'm going to investigate foamex.
alex, does the foamex compress much ? I imagine that over a large area it will not.