Yes, the one at Costco for $83 is definitely a good choice. Not sure what the difference between the two different Sams offerings would be. A GC2 is a GC2 as far as I know. I'm not a Sams member, hopefully someone who is has seen these 2 and can explain what the difference is.
As far as the box, I'd just go with whatever works. They have the same horizontal footprint as a group 24, so they'll definitely fit in your tray. They are taller though, so even though you could put them in group 24 boxes, they'll stick out above the top. Surely someone makes a plastic GC2 box? I also saw someone here fasten a stainless steel or aluminum cargo box to the A-frame that was about the right size and had a lockable cover. Really slick installation! I'll see if I can find the thread.
What I personally have been doing is just strapping them down in the tray without a box when we go camping. When I get home, I remove them and store them/charge them/maintain them in the garage. Someday when I get around to adding solar, I'll stop doing the "battery drag," but for now it works for me.