Normally, boxes that have integral hubs have smooth surfaces where they enter the box. It's odd that yours do not. I see one of the conductor's is a #3, so a bushing would be required. I can't think of a better way (that is listed) than what you have suggested. A chase nipple topped by a plastic bushing. Obviously, you would have to de-terminate or cut the conductors to do this. Another way that isn't listed but would provide protection is to squirt some silicone into the hub and hold the conductors away from the sides while it hardens.
300.4(G) Insulated Fittings. Where raceways contain 4 AWG
or larger insulated circuit conductors, and these conductors
enter a cabinet, a box, an enclosure, or a raceway, the conductors
shall be protected by an identified fitting providing
a smoothly rounded insulating surface, unless the conductors
are separated from the fitting or raceway by identified
insulating material that is securely fastened in place.
Exception: Where threaded hubs or bosses that are an
integral part of a cabinet, box, enclosure, or raceway provide
a smoothly rounded or flared entry for conductors.