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You might try some masking tape around the knot to get it temporarily snug.
If you get silicone in the 'hairs' would not be good, I think?

Well, lots of ways to do it, but washing the new knot first a few times and allowing it to dry has been my best tool to determine preferred loft as test-fitting. As it has bloomed (In 2-band...

I have a few which are vintage reknots. They came with Boar knots and are sometimes set higher than what I want my 2-bander at. If you don't have room to go deeper, an o-ring works. I have a few like...

It's a subjective thing. A handle which will hold a 24mm will hold a 22.
At same loft will be different brushes.Type of knot matters a lot, of course. Denseness, etc.
I try to get them to fit as...