I'm really curious as to how these are made and the effectiveness? Does anyone own or have they tried these?

2 Sided Vacuum Clamps Pods Vacuum Table CNC Machining | eBay

Are they simply machined flat with a hole drilled in he center then routed for a gasket or is there more to it than that on the internals? are they a single piece of material or 2 pieces assembled together somehow?

Adam,