I've never seen a 12ga bore guide, but in the event one's made by someone, I doubt it's needed.
Bore guides are to keep nearly-bore-sized, or flexing, cleaning rods from rubbing against the rifling @ the muzzle (usually) - and a 12ga bore (.729") is over 2x bigger than a 12ga cleaning rod, which don't usually flex much anyway.
A slug gun with a rifled barrel's still just a rifle, but bigger than most (.72 cal)
I just use a 12ga bronze brush in mine to get rid of any melted plastic from the sabots, then finish up with a bore cleaner on patches.
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