How do you draw accurate lines across round log at an angle?

I'm practicing cutting a crucks tenon and mortise, and I'm trying to cut a 30º angle on the mortise, however, I'm having trouble getting parallel lines on the round angle of the log. A flexible metal ruler would do the trick, if I had one, but they're expensive and I feel there must be other solutions I just haven't thought up yet. I can't find anything on the topic on Google either.

You don't need an official ruler to draw those lines. A strip of sheetmetal marked at suitable intervals would do the same job. Some aluminum flashing, even a strip cut from a smooth tin can, would work.

If you want uniform-width mortises, make the metal strip that wide.

Hmmm sounds reasonable. I'll have to find sheet metal somewhere, and I like your idea to combine the two uses. Thanks