It has been a long time since I last did any darkroom work but I read somewhere if you put a narrow piece of opaque tape across the front of the enlarging lens it would give you two images. When you focus the lens the two images will combine into one. At that point you will be in focus. I remember trying this and it worked.
The hassel was attaching the tape on and off.
I hope this is not my mind p[laying tricks on me but I vividly remember this working. I read through all 3 pages of this thread and am surprised no one else mentioned it.

Francis in VT