Portager-Think about a square gusset plates-diamond when welded on. Then on that gusset plate welding a short section of U channel-open end up-on the gusset plate-with holes drilled thought it to accept bolts that would fasten a section of tubing of the appropriate size for the brace between the two upper rack halfs might work. More than one bolt is going to be better due to the racking action....you made also want to consider using locking nuts for peace of mind...or lock washers at a minimum. It would be relativing easy to tear down for storage. And an alternative to square tubing is rectangular tubing for the horizontals, stronger too depending on the orientation...welding a chain link or hooks on the underside of tubing works good too, for the tie downs....Spray on bed liner has been a good performer on the custom ladder rack on the truck (and I have also seen it using by a trailer makers rather than paint and some folks are using it on the rockers and fender flares of their trucks-or powder coating-or if a forever system is desired make it out of stainless steel-which is stick weldable. Yeah I know stainless is of a higher cost....but think about this...no paint and it is forever-made the investment in rail boxes for the work truck after having carbon steel....the carbon steel had a second coat of paint and they perforated in four years....HOLY BOXES....the stainless in now 5 years old and looks brand new and does not crack like Al...aluminum. The milk truck now has 170,000 plus miles on it and is driven year round. Be warned that stainless is a little harder to work with due to it's cutting (and drilling) characteristics and it does cost more but it is forever! After having experience with it it is a no brainer for me...and heck you will the canoe trailer to that special person! ;) or at least the rack!