If I recall correctly Robert Vonk of Fotohuis used to sell a thing called a Versamask which was a metal easel with very accurate metal masks for 6x4, 5x7 10x8
Unfortunately only available directly at the manufacturer in Germany only. By buying a metal insert for the mask you can make in two steps a perfect black border (e.g. fixed 2mm) in the mask. Versamask is not really expensive but all frames are laser cutted.

Otherwise with a 25x37mm carrier from your enlarger you will have a more artistic black line.

Here an example of the Dunco 67 II 25x37mm glasless carrier:

"Ertsgebirge" Germany-Czech Republic on Rollei ATP1.1 - RLC 1+5 (M7 Summicron 2,0/50mm)

Miss Alissa in the Rollei Pan 25 test. Versamask black border (2mm) on Fomabrom Variant FB paper.
Rollei Pan 25, AM50 1+29. M7 + Summicron 2,0/50mm.