I'm not sure why you want an IO shield. To cut down the little air leak? To reduce EMI slightly?
Years ago I did something like this for a kit pre-amplifier that seemed to be picking up EMI noise in the phono section, and it worked very well. (We all remember phonograph record turntables, right?) I got some foil tape and covered the inside of the wood case top with it.
Maybe you could glue aluminum foil to some light cardboard (or use aluminum foil tape) and make the appropriate holes in it. Easier than drilling sheet metal. Scissors would work. The foil should be on the inside to make contact with the little prong fingers on the mobo connectors.
Aluminum foil tape can be found in hardware stores like Home Depot. Mainly it is use to seal furnace pipes/ducts. (Don't get the vinyl/cloth duct tape.) It is on the expensive side unless you can find a really small roll. It is also sometimes found in auto parts stores for repairing rust holes.