I find that my 23cII leaks light, so I've started to work on sealing it up. The seals I was asking about are atop the diffuser (if you have one) or condenser lens apparatus. When the diffuser/condenser slides in on the rails, there's a small gap, and light can leak out. I fixed this issue with some spare self-adhesive Velcro. I took the fuzzy part and cut it into narrow strips, which I then affixed to the condenser/diffuser as zsas shows in his picture.
Next I cut some soft foam to fit into the hinged lid, and used a large rubber band to hold the lid against the enlarger head. No more leaks from there!
I will now have to seal up where the lightbulb goes in (no biggie, just more foam) and will make a skirt as Mike Wilde mentions, except that my last leak is where the negative carrier slides into the enlarger.