Don't use plastic, you won't get enough heat transfer to chill the beer fast enough.
Don't use copper, it will give you off flavors in your beer.
If you can't afford a stainless coil or a plate chiller, don't build a jockey box.
Instead go with something like my setup.
For portable beer dispensing, a corny keg fits really well in a 5-gallon bucket from homedepot, with room around the outside for ice. Instead of a cobra tap I wanted to use a real faucet, so I drilled a one in a 1x6 and ran a shank through it. The 1x6 conveniently fits in front of the keg in the bucket (and there's still room for ice). The hose and faucet and gas and everything gets attached as normal. It's not the prettiest setup in the world, but I've seen worse and it gets the job done as long as you start with cold beer and the ambient temp isn't too high. I use this setup and drag a keg of Irish stout on nitro in a little red wagon to my neighbors house every year when his dad comes to visit from Dublin.
I have visions of something nicer and jockey box-ish, but it's low on the priority list for me.