DC the reason I asked about the size of your bed was your comment about 20mm aluminium being some kind of standard approach, surely the thickness of the bed would be proportional to the size? e.g. a 20mm plate would be overkill in 400 x 400 but not good enough for a 4m bed. I'm considering options for a 800 x 600 bed and one thought is to make a grid, with around 75mm spacing and top it off with a full 800 x 600 drilled and tapped plate. This suits me since I have loads of 15mm plate, maybe a similar approach could work for you?