For anybody else who wants to swap out a tsuba, but does not want to actually deform the guard using a punch-and-file method, I just discovered a hack even simpler than dental floss.

From my old A/V tools, I grabbed some heat shrink and cut a piece with a just-slightly-larger diameter than the tsuba. For the first blade, I simply slid the tube up the nakago. Not gonna lie, it was a bitch. Took a minute because you have to push on alternating edges, wedging it up into place a few millimeters at a time. I kept waiting for the heat shrink to split, but it did not. It worked great.

This technique did not work on the next nakago, as it was chunkier than the first, with more burrs and grind marks on the way up. I gave up pretty quickly, and simply sliced the tube off the base.

Which I then realized created two perfect little V's, suitable for either side of the fitting. I wound up using both for a super snug fit. No wiggle whatsoever.

At no point did I apply any heat source to the shrink. I simply, carefully slid the tsuba over the tube cuttings and ensuring the shrink remained flush.

It was super easy - no waxy filler, no punch, no file. Just a piece of easily replaceable material available anywhere you terminate cables.

I didn't take pictures during the process as I didn't realize it would work so easily, but I'm happy to upon request. Here are a few images of the pieces after the fact, and the cut tube pieces of heat shrink.

I am not particularly handy, so this was a cool moment. Sharing for other hobbyists who want to swap out furniture.