I don't know what the OD of the 357 is I just got mine back but williams makes a 20ga shorty ramp that is 0.690 OD in 1/8, 3/16, 9/32, 3/8 hights.
I'll try to check mine this weekend I would like a fire sight for it too. New ones have williams sights on them so hopefully screws will be the same. Also got a Manlicher forearm when I got my barrel :grin: