PCBgogo

For various cost and inventory reasons I need to bring the machining (hole cutting) of the stock plastic enclosures we use in-house.  I have all of the pieces in place (mostly a CNC mill), except for production workholding/fixturing.
I've been testing just by using a machining vise but that's a pain in the rear and doesn't hold these vertically all that well, and isn't that fast to change things over.    I'm thinking about some pneumatic or mechanically driven clamp, but don't really know where to start.   I've seen these types of jigs in use, but haven't built one myself.   My goal is to have something which we can drop an enclosure into and the fixture takes care of ensuring that it is positioned and held correctly on the X and Y axis. 
I've been perusing various workholding/fixturing sites but can't seem to come across anything which feels right.   I've also been looking for various books/instructions to give hints, along with perusing youtube videos - but most people are milling things out of blocks of metal which isn't really the same unfortunately.    So far, I just don't feel like I've found enough information about what options I have.   I'm hoping that someone on here has been through this and can point me at least toward the resources I need to figure this out.
I'm pretty good at building something out of available resources, but in this case I just don't seem to have the right set of resources to even get started.
Ideas?