It could probably be done as a fabricated piece- a steel cover plate with a clearance hole and bolt pattern and a piece of pipe with a welded cap, or just a pipe cap of appropriate size could be welded on. http://www.st-pipefittings.com/ANSI%20B16.9-end%20cap.pdf
A skim cut on the back side would leave it flat and smooth enough to seal up.

A water/laser jet or plasma cutting table could give you a good enough precision and finish to use it direct without any machining, or could use the vacuum pump as a drill template before welding it up. Don't have holes cut hot (laser/flame/plasma) that you mean to drill in any way, because they'll be hardened by the rapid cooling in the process.

This is less than a day's CAD work with good information to start with.

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