I was able to use the Dremel diamond bit to bevel some AN glass I cut down from 5x7 for my 4x5 enlarger. I do have a spare reflector and believe I would be able to enlarge the opening with that Dremel bit.

Click image for larger version.  Name: Dremel AN Glass.jpg  Views: 22  Size: 31.5 KB  ID: 201401
Click image for larger version.  Name: Old and New durst Reflectors 2.jpg  Views: 24  Size: 61.9 KB  ID: 201402