A contact adhesive is fine but don't use an ordinary household glue. Most OEM bellows (since WWII) use Evostick or equivatlent to glue them to the frames.
The Beseler 8038 3¼” extended lens cone fits both the 23C and 45M series enlargers. This allows smaller than usual prints by placing the lens far enough from the negative to focus.
If you can’t find the extended cone you could make one. The main plate is simply a sheet of aluminum 1/8” x 4” x 4”.
You’d have to fabricate an extension to fit the plate that doesn’t interfere with the receiver at the bottom of the bellows.
As Post #6 suggests, a shorter lens (within reason) will fully cover the negative if the print size you want places the lens far enough from the negative.
Thanks to all for their suggestions. I used what was locally available (E-6000) and it sure has stuck well. Thanks also for the suggestions about different focal lengths and cone lens boards, but the problem was fundamentally that old glue had lost flexibility since around 1970 (I seem to have lost flexibility too since that time).
I finally purchased my first enlarger today!!! It's a Beseler 23 C II in almost perfect condition. Now that it's home and I'm playing with it, I noticed I have the same problem as the OP :(
But after doing a search and seeing that it sounds like a common condition I now feel like it's nothing to worry about and I'm not freaking out :)
I picked up the enlarger with a 50mm lens and neg. holder, a Time-O-Lite M-59, a 5"x7" Premier safe-light, and a canister style one. I paid $125. I feel I got a nice deal, and I'm itching to get the few other things needed to get started.
You got a great deal on that Beseler. Check out the thread on aligning it using a laser level....
and here is a link to another Bessler site that talks about mods n alignment...
Alignment is everything no matter what enlarger you own.