For our Sasuke's collar we ended up using some bug netting to stiffen it up, but still have it able to bend. (Since I can't imagine anyone would be too happy to have a stiff tube around their neck.)
It worked pretty well and wasn't in the way.
We just took an existing shirt and cut off the bottom and used it for the neck. Then we attached the bug netting in between the blue and white fabric with no edges of the netting on the outside so it would not scratch her neck. If you just get a really large shirt it should not be a problem.
I wouldn't suggest cardboard. My friend did one like that and the cardboard broke halfway through the convention with no time to fix it.
(I also am not too sure where to find bug netting, ours we had conveniently lying around the house...)
Sasuke Collar Example