depends on the level of perfection needed. I bought a miter jack from England a couple of years ago, it's close to dead perfect and was cheap enough that it made no sense to try to duplicate it or deal with a type of fixture that may not be good for large and small pieces.
A With some hand touch, minor tuning can be done freehand either with planes, floats, gouges and chisels or with a glass sandpaper lap where the sandpaper is PSA roll (no part of it can come up and dub work).
The ability to tune things freehand to fit will save time and will also allow you to get out of absurd spots that you shouldn't have gotten into, like this:
https://i.imgur.com/lFcCLcI.jpg (notice that the miter meets in the center of the front moulding and not in a neat corner. I had to cut the flat part of the fit fat and then clean it up by hand, but it wasn't that difficult to do - far easier than it would've been to come up with some combination of fixtures and keep test fitting.