Iain: My personal choice for a top plate is laser cut stainless steel,
4mm.

I considered that but that is HARD! I don't fancy working on 4mm
stainless - don't you find that it blunts your hole saws and drill
bits???

Using a hole saw in my drill press on 4mm alu is time consuming enough
when the diameter gets to 30mm and above (even 25mm takes a little
time) - I can't imagine what it's like on stainless. Andy