I also think carriage bolts would be to big. I ended up using screws with the head diameter that looked right then filed and filled the driver hole with epoxy. The down side to this is, once installed theres no removal without drilling out the epoxy. I'd have to go look at the plans but I don't think the box fasteners went all the way thru the trail. I'm liking the build pics they should help out future builders. And Merry Christmas.
“I won't be wronged. I won't be insulted. I won't be laid a-hand on. I don't do these things to other people, and I require the same from them.”
― John Wayne "The Shootist"