He installed a fatter (thicker rubber, slightly more circumference) o-ring on the lower cartridge seal that sits at the bottom of the fork leg. This allowed a tighter fit of the cartridge into the fork. Otherwise, the cart feels a little loose in there, like it’s not fully sealed at the bottom.
There are holes in the fork legs as well, those need to be plugged. We used rubber stoppers from the inside of the fork but I would probably tap the existing holes and put a bolt in it from the outside with some loctite. This would prevent any oil at all from leaking. My suspension guy didn’t think plugging the hole was necessary with the better fitting o-ring but as a safety measure he plugged in the event that the bottom o-ring fails and lets oil leak past it.