Damn it! I have an absolute stack of Rolls Razor gear, including the mirror and the soaps (both dish and stick) but the brush has eluded me.
If the hole is 23.5mm ish, I'd say go for a 22mm knot.
It is actually pretty easy to do yourself with minimal tools. What I do is use a sharp kitchen knife or a single edge blade to cut the bristles off as close to the handle as possible. From there, I Dremel out the knot. Take the new knot and drop it into the hole. Look at it from all angles and go deeper if need be, or pad it out with a single penny to raise the knot up. Happy? Drop from epoxy in, the optional penny and the knot. The excess epoxy should be enough to just go up the sides of the knot but not spill out over the top. Make sure it's level all around and let it set. Enjoy the next day or the day after ...
What kind of knot are you after? Badger? Boar? Synthetic? I have some white boar surplus to my requirement, but I think they're 20mm and would look a little skinny in the hole as well as leaving a bit of a gap.