Thanks guys - that's just the kind of input I was looking for.

Sounds like I need to bite the bullet and make something - and then see how it works for what I do. Rather than try and overthink it all first.

So I'll grab a bit of ally plate - a little oversize for my mill table - and drill/tap a grid of holes at 1" spacing. And I agree that M6 is likely a good choice here. Then it just needs holes adding to bolt through for T-nuts, to hold it to the table.

Then I should use it in anger, and see how it works. I'm sure a Mk2 will happen at a later time, but this sounds like a great way to learn what I really need!