Maybe a technician specialized in optic constructions, like this one:

http://www.adrianololli.com/

would be able to propose a solution and cut some optical glass of the right dimension with a cost which should be - I imagine - much lower than asking Zeiss, Schneider, Schott etc. to do the work. It would also give some peace of mind in case of accidental breaking.

The reason why the glass sold with the enlarger is "simple glass" is the same why break pad of my bicycle are in rubber, or why the springs of my motorbike are simple (non-progressive): they are good enough and better that that certainly exists but raises the cost considerably. If cost is of secondary importance, "customising" can certainly improve the quality of most products. Changing the glass to the enlarger is something like putting light alloy wheels to a car. Big expense for relatively small performance gain, but gain there is, and if that small performance gain is important...