Im not really an A Frame kind of guy bingosucks but if I was..I would certainly make mine rather than buy it.
I have not given it much thought..but looked in my shed to see whatI would use if I was going to make one..and I kind of like your idea on copper plumbing as its easy to bend and join. However as mister p mentions..its probably too soft. A wave would bend and buckle it, plus it would look like copper plumbing.
Think I would go for a composite pipe. I found PVC drain pipe at 21.5mm diameter and it has a nice shiny finish. Obviously on its own..not strong enough..but stuff it with 15mm copper pipe..which has in turn been stuffed a wooden dowl..and you are starting to build rigidity very cheaply. Use the copper pipe solder type joins ..and perhaps car dent repair stuff to join the plastic pipe so it can be sanded smooth to the frame profile..then paint it ? Seal the ends with epoxy so water cant get to the copper pipe ?
Just some quick thinking..but worth trying some experiments. Please post how you get on with it too..other might be interested