You can buy an inexpensive wooden miter box with 90 and 45 degree cutting slots at a hardware store. Use a hacksaw and work carefully, and clean the edges with sandpaper, emery cloth, or a file. It doesn't matter if the 45 degree bevel isn't perfectly aligned because the wedge nut will just follow the bevel.
I can't see how the stem could be secured reliably in one of those. The ones I've seen have plastic pegs to hold wood in place. I would be worried about the stem rolling and messing up the angled cut.
