You might want to ask over at sejw. Solders real easy. Typical plumbing flux is zinc chloride. proly wouldn't want to use any acid fluxes.
You could also consider not soldering, but doing something like what Idid in a closet: Plywood walls, 1/8 x 1" strip over the seams. Looked really cool--well, at least in my closet! This is for larger areas tho. Soldering edges, corners, etc. sort of leaves you no choice. May consider silver solder. Or even brazing! Or epoxy. :) :)
But here's the bigger problem w/ copper, as a decoration. Yeah, it's beautiful, but to *keep* it beautiful is constant maintenance--unless you just want the patina, but then, as a roofer pointed out to me, you gotta really like *green*!! The solution is to coat it, somehow, but that always struck me as cheating, and ultimately temporary/vulnerable.
An alternate solution is to use bronze sheet. Will knock yer socks off, it ages *really really nice*, w/o patina, fwiu. Different shades of bronze, as well, altho I don't know if you will have many options when buying sheets. Not sure of the available gauges, either, below 1/8. In machining parts for my Folly, I use brass only for practice, bronze for the real parts. Big diff.
Keep in mind there are all types of copper as well, roofing copper being the cheapest and softest. But you can buy all grades/hardnesses in sheets.