My plugs are all cut from dowels. May have one 'official green plastic' Remington. I made one from a limb for a buddy who ended up in the field with no plug.
Keep the Mod bbl. Watch for an IC. Doubtful you'll need more.
Good screw in choke work is not cheap. Cheap screw in choke work is ... well, cheap! The ubes themselves are not cheap either. If you just have to have them then just bite the bullet and buy a bbl with them. It will come with three tubes. But may cost you almost as much as the 870 did! Cabela's, Midway, Brownell's, ... oh, Remington.
I have several bbls with screw in chokes .... don't even remember last time I changed one of the damned things! Most entertaining (and aggravating) thing at a sporting clays range is watching guys madly changing tubes! Then saying, 'guess I should have kept in the xxx'. LOL!!!
The best thing screw in chokes have done is make standard used guns and bbls cheaper!!