On my .500 Phantom I took the gas block from midlength to CAR length. I tapped the midlength port half way through and used a plug screw to block it off. Red Lock-Tite and staking the straight screw slot on both sides and that screw is not coming out and since it was bottom tapped only half way through, it cannot go deeper. It’s been that way for years and is barely noticeable, especially since it’s under the handguard.
By the way, it’s .925 at the old port (turned down slightly from .936 originally) so with a .510 bore, I have roughly .207 wall thickness so the ⅛” plug screw works out perfectly for me.
If you really want to go cheap and easy, make a steel ring with rounded edges, like a wedding ring, about a half inch wide, with an I.D. about .002 under barrel diameter at the port. Heat it, slip it on, and a little touch up on the finish and done. You could even make it look like a barrel band sling swivel attachment, like
this. or
this. It would not look like an afterthought then. Plus, the second one already has the correct ID but would likely need to be soft soldered into place. No need for silver solder.