Randy:
One of the reasons I haven't pursued getting the Lyman Foster slug to shoot better is that I hate my mould! It is the most stubborn, cantankerous mould I have ever used! It has always cast poorly and the slugs tend to stick on the core pin. I don't think I ever took the time to polish it but the angle isn't enough for decent release anyway... in my opinion. I've made moulds that cast better and make better slugs... at least they are more accurate so why bother?
What you might try is to use a chunk of 12 ga. barrel and a flat or cup nose punch to swell the Lyman slug to bore diameter. I hadn't actually thought of that but one of the fellows whose handle I forgot posted that recently and says it works pretty well. The other was SluggerDoug who made a "knurler" from helical gears to impress "rifling" into the slug and bring it up to bore diameter. SluggerDoug also said that made the Lyman slug more accurate. Basically, get it to bore diameter and accuracy improves.
My TC slug gets knurled to bore diameter for my single shot which runs 0.733" where the slug casts at 0.729". I haven't tried them in my Slugster yet but will. It runs 0.729"/0.730" so should be about perfect fit. Blood Trail tested these and they did okay from rifled gun but nothing spectacular.
A solid lead slug from a metallic cartridge is likely the best bet. I'm with Cap'n Morgan on that. Cartridges (or chamber filling "adaptors") made to reduced volume and that will align those slugs should remove some of the inherent inaccuracies of components that we are all fighting. A guy would not want to be leaving those in the field after shooting though so maybe not so practical for hunting using repeating guns. You'd definitely want to be keeping an eye on ejected hulls!
You could use brass, copper or zinc in those Hammerhead wads. They are actually a pretty nice set up and offer some good options for slugs and obviously they are accurate. I wish they made a smoothbore version. I am a bit surprised they don't work well in smoothbore but apparently not.
A guy with a lathe could make his own non-discarding sabots using nylon, polyethylene, Delrin or likely some other plastics but it would be a bit tedious unless you had CNC or made tooling to get consistent turning and boring sizes. I still like AQ design with solid tail wad rather than cushion leg... or Breneneke Classic.
One man's opinion.
Longbow