We decided that the stock seat belts installed in the rear seat of our RB-50 layout were not safe for our two kids, age 7, and 9. Only one side had a shoulder strap, and it was pretty jenky. So we bought these 4 point seat belts to replace the right and left side of the stock belts.
4 Point Seat Belts - Push Button Buckle
These belts require you to drill one extra hole per seatbelt for the 3rd point (middle) of attachment. I ended up having to drill a total of 3 new holes. One for the 3rd hole on the right side, and two for the left side. The left side has a jenky shoulder strap belt and I wasn't able to reuse the anchor point due to it's positioning.
I ended up having to remove the bottom portion of the bench seat as access was prohibited from the back due to the way the water line was run for the shower (see pic). It would have gotten messy I think to try and cut the hole into a U shape allowing the partition to lift out and expose the anchor points for the seat belts. Although I still removed two screws from the partition to move it away from the seat belt anchor points to allow for drilling and installation.
These seat belts work out great. I'd rather there be a disconnect buckle from the third anchor to allow the belts to easily be pushed through to the bottom of the bed. Instead, I must loosen the belts, and slide them to the side when transforming into bed mode. Not a deal breaker, but theres a bunch of belts to deal with. But you can see from the picture that the seat does transform into the bed nicely still.
Also, we searched for a good solution for a head rest for the bench seat to help with whiplash in the event of a crash. This little back support and head rest works out great for our tall 9 year old.
Now wife is happy, and kids are safe!