The way I cut mast squares: If you have a diamond file and a razor saw, you should be able to square the mast heads with patience. First measure the depth of the squared end...mark that point around the dowel. Determine the size of the square, and mark on the end of the dowel. Using the razor saw cut a notch at the depth line down to the depth of the edge of the square. Place the dowel in a hobby vise end up, use the razor saw to cut from the end to the cut you made earlier...this will nicely form the first edge of the square. Repeat for the remaining three edges. Use the diamond file for final shaping. Anyway, there are ways to get a well formed square with simple tools. Hope this makes sense...hard to draw word pictures! Oh, I know a master builder who only uses a sharp no. eleven blade and makes square mast ends perfectly. He is way more skilled than I!
I use this method too and it doesn't seem to take a lot of time at all.