I also stayed at a Holiday Inn Express one time.
Here’s a detail of how I inserted the rectangular tubing into the existing body hoops. The technique was to have the rectangular tubing extend as much as possible into the old structure, then set the flat bar shims in flush to keep everything snug. Once flatbar and square tubes are clamped on upper and lower, it tends to force all the assemblies back into square.
After they are all pressed I to place, I welded .125 inch angle iron “ears” onto the rectangular tube between the upper and lower original structure. I figured it would help with fore and aft flex,as well as providing an easy flush surface to rivet the body sheets to.
One thing I did was attach VHB tape strips to each flat surface then stick the sheet metal to it. With that technique I was able to press and hold the metal in the right position, then peel and stick the second side of theVHB tape. The panels just hung there for the most part until I could drill a hole or two for clecos.