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.