There's a much more accurate way - I often do the "trace on a paper higher in the enlarger path", but this gives halos for extreme changes.
I got some register pins that are the same size as a 3-ring-binder punch. I punch the edge of the paper and register it to the baseboard. (Once you do this, you cannot touch the focus and even changing the iris can knock it all off). Then I punch a sheet of ortho-lith film, expose it, and get a high-contrast positive. I can contact print this to get a negative if needed, and use reducer to clean things up. (You can do all of this on the negative plane if you have a registered negative carrier, something I still need to do).
This is a crappy phone shot, but just this week I replaced the sky for this lith print:
This is the paper, registered to the baseboard:
Here's the mask (you can see a register hole) and an 11x14 neg I made by contact printing an RC print onto litho film - I toned this to beef it up, too:
So I exposed the paper for x-time with dodging and burning, laid the mask over it with the cloud neg taped to that where I wanted it, and then exposed the clouds with more dodging and burning. Worked well for me, though I'd much rather be doing this all on the film plane - once I take this setup down, it will never be accurate again, but with registered negs for the carrier, I can do it again and again, at any size.