That’s a tough one, but I think the answer is just a removable bar, or half loops.
I don’t remember any members ever doing both types of belts. They either had three pointers or the full harness, but not sure if anybody swaps them out between trips.
Usually they have one type and stick with it, but no matter what, the full harness is usually troublesome.
I think I have seen loops instead of a crossbar before. That still blocks access from behind the side of the seat, but leaves the area between the seats open.
Either type is usually welded. The typical removable type leaves some sort of an anchor point that’s still going to be in the way for side access. Seems like between the seats is still going to be the answer.
Good question though. Don’t think I’ve seen a good discussion about this in a long time.
Good luck!