Ian
I Know what you mean my current bed is 400x400 (Or was till I chopped the corners of for the Delta and that is 6mm which is fine.
If you remember we spoke about the temp issue and I pointed out that I drilled my plate to put the thermister closer to the top surface tho out on the periphery of the bed the heater I have ordered I have asked for there to be a 20mm diam hole at the centre of iut to allow me to get the Thermister as close to the top of the center of the bed as I can (I have some that are mounted in a M3 Brass carrier so I will drill and tap the center of the bed to take one of them.
Dont think it would be easy or even possible to brace the center without beefing up the whole bottom section especially taking into account the silicon pad?
Doug