I've used three different vacuum setup on different machines,none of them a Shop Sabre.The first was one I made using the technique you describe of cutting a grid into the underside of a sheet of MDF and then machining a groove for a neoprene sealing strip into the top of it with holes drilled through to connect with the vacuum channels. It worked reasonably well and we moved around the sheet with various jobs as we had a supply of small plastic top hat type plugs for the unused holes.The other systems I have used had plastic or resin sealed MDF on the machine table with sub-divisions to create six or eight zones that could be individually isolated,and needed to be because small parts need lots of vacuum to remain in place.The plastic was hollowed out with lots of ribs left standing in each zone to support the MDF spoilboard.With a 25Hp pump you ought to be in good shape if you can ensure enough flow capacity over the table of the machine.