A conversion neck is one that can convert an instrument from one scale length to another with no other modification. A common conversion is 34" to 32". By overhanging the 20th fret, or leaving it off, you can use the stock bridge location on the body, just adjust for intonation. Such a neck will work on any standard bass body. A conversion from 34" to 30" requires overhanging two frets, or losing one or both.
I made this little mitre box. It goes in 5-degree steps from 0 to 45 degrees. I don't know if that will actually be useful, but it gives me something to play with. Although I made it with the saw shown, I would use it with a fret saw, which is narrower and runs in the slots nicely. The important thing is not that the angles are super accurate, but that they are repeatable. I think.