Several years ago I bough an 18" x 12" vacuum hold down chuck for use in engraving on my Interact. Made by a firm called Debece it comprises a sandwich of an aluminium core with milled airways, a lower mounting layer of some white plastic material, and an upper layer of the same white plastic with a quadzillion tiny holes in it each at the base of a 4mm drilling.
I never used it in anger as it was warped when I got it - about a 4 mm 'hump' from side to side so pretty useless - I honestly cannot remember if they refunded me or what, but it's sat in a box until just now when I dug it out when looking for the Tapmatic Drill Speeder. I think that they are out of business as they don't pop up on a Google search. Measuring the hump again it seems to have settled down to about 0.5 mm but as it 'humps' rather than 'sags' the hold down clamps don't straighten it to the machine bed.
So I though that it was time for some ingenuity. I set it up on spacers on the bed of the manual Bridgeport so that the extreme edges of the short sides were suspended on bars. I set up a dial gauge below it to measure deflection, and put a beam parallel to the bars and lowered the mill ram onto it until I'd bent the sandwich 3 mm in the opposite direction. Left it there and had lunch.
Releasing the pressure it sprung back, initially to about 0.5 mm in the opposite direction to the original hump, then slowly over the next hour crept back to dead nuts on flat!
No doubt it will creep again over time, and I may try bonding it to a thick alloy tooling plate, but at least it's now usable

Andrew