Assuming it's made of PVC, any auto parts store sells Schrader valves, the kind you fill tires with, and you could drill a hole and carefully thread the valve in. If you apply even pressure while screwing it into the hole and take your time the valve itself will tap the plastic. If you put it right at the top of the apparatus, you've just got to push in the valve stem with a screwdriver or fingernail and you're golden.
ETA: Most of these are the same size as 1/8" NPT fittings, but they do not have the taper. If you happen to have a 1/8" tap lying around or want to buy one, you can do a very nice job of it.