negative carrier for 828 ...
Beseler 45 enlarger.
If you can't find an 828 you can also file out a 126 (26.5mm sq) carrier to 828 (28x40mm). 126 carriers aren't all that hard to find, and sell (should sell) for close to nothing.
If the idea of spending time with a file doesn't appeal then a machine shop can do the job for you for not that much money ... explain that you aren't looking for a high-tolerance job. In lieu of other instructions a machine shop will mill it out to 1/1000 of an inch or better and charge accordingly.
A cost and labor saving tip is to drill 4 small 1/16" or so holes at the corners of the new cut-out. You then file or mill from hole center to hole center. That way you don't have to square out the corners when you are done.