I wanted an access hole for the docking cable. The perfect width of slot (to hold the New iPad or "iPad3") turns out to be about 3/8" so I fitted my drill press with a 3/8" Forstner bit
. Drilling from the bottom avoids damaging the topside edges of the slot. This drilling can also be done with a regular hand-held drill and twist bit but requires a firmer hold and more care.