Nothing like a challenge .... To be more exact, 18-25mm Marine Ply, used in "I beam" configurations... maybe with some Aluminium parts in the gantry. The base will be Ply. If I had a cast iron bed plate, hold down clamps and a MIG set yeah I'd make a steel one... but I don't, neither do I want a warped heat distorted frame to shim up and true later.
Aiming at around 1200 x 800 bed size to achieve 1000x600 cutting area. (don't want loose all my garage, as it's only a single!)

Tricky to get some of the stress and modulus data for Ply, anyway current scheme is suggesting via FEA to be 0.02mm deflection with a 50Kg load about the motor mounting platform. (circ 12Kg mass, which I can reduced). Not a bad start I though.
Can't image to see that load on a cutter, but it helps to pin point the week areas. Any idea what the lateral forces are on a cutter?

I'll scheme up an Aluminium gantry with some extrude section 8, 160x80, and end plates to compare, then work out the easiest build sequence. That will required some machining to be contracted out... Some perhaps someone in the Midlands will be able to help out :)

I'll post up a design thread when I've got a bit more data and detailed laid down...... appreciate all the comments, interesting and fun stuff.