Why not Briley SideKicks? I know they work for 2-5/8" Parkers because I have a set for a Damascus hammer Parker 12. Briley will ask you what kind of gun they are for and will engrave that maker's name on the SideKicks. Something about rim depth or extractor configuration, been a while since I ordered mine.

https://www.briley.com/p-52225-side-kick-20-chamber-inserts-complete-set-20ga-28ga-and-410-bore.aspx

Another option might be chamber inserts. I shoot 20ga ChamberMates in a Brit 12 twist hammer gun with short chambers and they work with no modifications. GaugeMates told me (when they were still in business) that their inserts could be shortened, if necessary, with no ill effects. Browning chamber inserts are still available, but they are not as convenient as the ChamberMates and GaugeMates.