"Box configuration? Pictures/links help."

I'll bet its the Wen seat yoke - that does sort of box in your head  -

while I don't particularly like mine, it does the job and has never loosened up or slipped on the seat frame - likely, with a lighter boat it might be that I'd find it not too bad - my vous is 58# spec weight, and add two floatation bags, bag cages and a spare paddle tucked in behind, its more like 65 pounds

I also have on that boat a strap yoke from Mohawk, which works well enough for shorter carries and would be worth considering for a lighter solo boat - not an option for a rental though.