I tried this a few days ago, it works pretty well. I put the... um... I forgot what they're called, but anyways, I put the thingies in the freezer for a little while, I then cut them to the right size. Then I took a soldering iron and made the holes, and immediately pulled the shrink tubing through. That way the shrink tube will stick to the... thingies.
Before I did any of that though, I put the shrink tube over a small screw driver that was the same thickness as the "tube" on the earphones, and applied heat.
The seal was great, but unfortunately my left ear started to hurt after an hour or so. I tried the yellow Shure foamies, they feel better in the ear and my ear doesn't hurt even after like 5 hours of use, so that's good.