I had a chance to try making some cork stoppers in preparation for my TM's arrival.
Lacking a dremel, I tried to hollow out the corks entirely with my drill. I started with a pilot hole using a 1/4" bit, then widened it with a 3/8" bit, and finally followed up with a 1/2" bit.
One problem I had while doing this was that the 1/2" bit and the cork do not play well together. The smaller sized drill bits made relatively clean holes, but the 1/2" one would begin tearing chunks of cork out if I wasn't careful:
I had originally figured that a 5/8" bit would drill a pretty good approximation of the mouthpiece diameter, but I didn't actually check to see if I owned a 5/8" bit.
I'm not sure how well a bit that large would actually work on the corks, since it would hollow out so much that I think the cork might lose structure and just be torn to pieces. Then again, I'm not much of a handyman, so perhaps someone with better technique could pull it off.
A 1/2" doesn't create a large enough hole on its own, and I found that trying to force my RBT stems (which I'm using to estimate the TM mouthpiece size) into the 1/2" hole would just destroy the corks. I needed something to widen the hole a little more.
I tried using the drill to widen the hole I'd made by swirling it around the edges of the 1/2" hole. I found that this would either tear out more chunks of cork, or not really widen the hole at all. Luckily, I found a little hand-file in my toolbox whose width is almost perfectly the same as the RBT stem diameter.
(You can see my various mistakes on the top right)
I gently forced the file into the 1/2" hole and then used it to sand the hole until it was wide enough. Not only did this make the hole wide enough to fit the RBT stems, but it also cleaned up the chunky edges from the 1/2" bit and smoothed out the inside.
In the end, I got 4 usable corks and botched maybe 11. I think I can get a better success rate now that I know what to do.
This was actually not such a complicated process. I'm not a DIY-oriented person at all, and this was about 40 minutes of experimentation. I think these corks might work well to protect the stem in my pocket, but I'd hesitate to use one to hold a loaded stem outside the TM. The bottom of the hole in each cork is not flush, nor is it smoothed by the file. I think the ground bud might easily come loose and fall into the cork if not packed correctly. Maybe that's something to work on today...