It involves 83mm heat shrink, which you can buy here, and a hot air gun.
- Cut to size, about 10mm or so outside the edges of the board
- Cut a slot out for the HDMI connectors (or audio / composite if you're so inclined)
- Apply hot air carefully so as to not distort the tubing around the HDMI cable
Pictures here (I failed to cut the tubing long enough so ended up layering a second one on: