Here's a couple of simple vacuum hold downs I made for the CNC router.
The first is about 4" x 2" by ¾" thick acrylic with a single passageway in from one edge and up the middle. Four magnets are recessed and glued into the bottom so it can attach to a heavy steel plate which sits on the bed of the router. A gasket can be made easily from a piece of thin foam ("fabfoam"). This was only to hold some foam for hot-wire cutting, so the cutting forces were pretty low. Yes, that is a plug on the underside of the block, since I mistakenly drilled right through when I was drilling the vacuum port!
The second is made from a piece of 16mm MDF sheet. I first routed out a recess and some supporting pillars, then painted it with a few coats of emulsion to try and seal the wood. Another foam gasket seals the outside, and a vacuum line is shoved into the edge of the wood to communicate with the small slot at the top. This was used for squaring up 20cm square pieces of 16mm MDF and held them very securely, despite quite a bit of air leakage through the wood (although I'd painted the hold down itself, I forgot that the workpiece itself is porous).