I had the same issue with my darkroom which is space limited; 4 feet by 7 feet. I had exposed wall studs inside the darkroom, so I cut a 2x12x55 board and mounted that between the wall studs using a very accurate level. The top of the board is 18 inches above the table, and that allows a maximum size of about 20x24 although the intent was to do 16x20 prints with some cropping. I mounted an old Lentar 6x6 and a Beseler 67 dichro head on there, and that was back about 25 years ago. Sad to say the darkroom hasn't seen much use over the last several years, but that is going to change as I get back into alternative process work.