I came up with this one years ago to help me set lathe tools quickly and easily.
It's made from an old square, I got this one with some old tools, it wasn't terribly accurate and I have several nicer ones in this size. An an aside I believe this to be homemade, maybe a school project from long ago.
This is so simple to make it you can do it in literally a few seconds.
Take a sharp scriber or lathe dead centre and put it in your headstock. Place the square on the bed of the lathe (or cross slide if you prefer, I use the bed as I have flat ways) And use the scribe to scratch a line into both sides of the square "blade".
Done, that's it! Your tool is ready to use.
If you have a gap bed lathe and can't get close enough to the headstock, lock down your tailstock and use this instead, it will be plenty accurate enough I am sure.
I added a picture, though you probably don't need it. You should be able to make out the scribed line on the blade about halfway up.