This is going to sound kind of rambling because there is a lot to say. Round or Raw base depends on the type of horn you are making. Round plugs are usually found on Factory and shop made horns. Most good horns can be rounded using a sizer, I would recommend, drilling and shaping your horn before you work the butt. Even if you have no lathe you can still round the base using a sizer and fit the base. Flat base is easier but a domed one, even a Beehive style can also be made, a second piece of wood can be carved as the strap holder. Once you cut the base and make sure it is Square trace around the horn, this can be done Raw or Rounded, make a witness mark on the base and horn, cut just to the line. If you have a disk sander it will help greatly, set the table to about 10 degs and start sanding that taper on the base, don't loose your Witness mark, once the base starts to fit take a soft pencil, like an HB, and color the inside the horn for about 1/2", press the base in, remove and cut, file, or sand off the pencil marks, it is slow work but works. No disk sander, Rasps, Files will work or if you have a Band Saw the table can be tilted to the 10deg or so mark and the base cut, then the pencil work starts. Speaking of Baseball bats, once you use it to round the base you could just cut it off and use it for the Butt plug, Bowling pins are another option but they take a bit of work to get the finish off. Also look for Branches, Apple Trees, Black Cherry all work well, maybe some in the firewood pile. This horn uses a branch, the entire horn was made with a knife, the base was carved to fit, not real pretty but functional.