Originally Posted by MattKing
Tape mat board or something similar to the movable arms in order to ensure that light is blocked in the area outside the arms.
Or use a mask at the enlarger stage, which gives less stray light. I doubt it matters much, but it is a second option. Some enlargers even have masks built into the enlarger head, like the Leitz V35.
"Often moments come looking for us". - Robert Frank
"Make good art!" - Neil Gaiman
"...the heart and mind are the true lens of the camera". - Yousuf Karsh