I forget the exact size, but there is a hole saw you can use that will easily trim the horn to an exact fit with the valve base. Don't even need to put it into a power drill, just rotate it by hand.
It's been years and years since I have installed one of those valves on a horn, but I think you will need to do a little file work so the valve will not interfere with the horn. Might be wrong, but that is what I recall. Sort of.
As far as the baseplug goes, I think leaving the horn natural shape would be best and look more appropriate. I have always liked using walnut for that, probably a 4/4 board epoxied onto a pine inner plug. You can either use a round over router bit to shape it (1/2" works well) or just leave it flat. Either way works good.
Have fun with the build, guaranteed it will look better than the pathetic POS that was my first horn.
John (bigsmoke)
