Get a pad of the same type as the rest on the horn. Punch a small hole right in the middle of the area within the tonehole rim when the pad is closed. I specify this carefully because some of the cup/ tonehole relationships are not centered- and that can be difficult to correct on the smaller cups where there's not much to work with for side to side bending and, of course, back to front alignment change is quite a pain. Just center on where the pad comes down- unless things are really out of whack the small hole in the touch will be in that area somewhere.
Having punched the small centered hole, take a cotton swab (e.g. Q-tip), smear it with a small amount of some suitable sealant from a tube - the sealant for tub/ sink use works fine, and insert the end of the swab in the hole and twirl it around so that the sealant glues down the edges of the pad surface and protects the pad interior from the gross water soakage that would occur if left unsealed. I'd make a point of immediately setting and seating the pad before the stuff had even set- just to ensure a good (light touch) seat- if you wait the stuff might conceivably harden up (though the silicone stuff remains flexy) with the pad surface distorted.
All that said, my bass clarinet did just fine with an unsealed hole punched in the pad for about twenty five years until I redid it recently (along with all the other pads).
