Mostly Off-the-Shelf (or at least quite easily fabricated) Rounded Square Tubing?
You didn't specify what you considered prohibitively expensive, so I would suggest something like vinyl fence posts. They come in sizes close to what you need, and you get quite a lot with one purchase, giving you room to make mistakes and experiment.

However, finding your exact dimensions is going to be difficult. To do that, I think you'd want to make your own. The cheapest and lightest (since you'll be carrying this around) way I can think to do that is to get some lengths of hobby wood. Balsa is cheap and light, but absorbent so it needs paint primer. Basswood is more expensive and a bit harder to work with, but is far less flimsy. I'd construct the tube out of 4 pieces, and probably glue them to blocks of styrofoam at the ends and middle. The foam will give the wood support and make it easier to glue the edges of the wood together. 

Then, I'd just sand down the corners of the "tube". It'd be time consuming, but not too bad, since balsa is so soft (basswood a bit less so).

It's also possible to just use styrofoam, cutting sheets or blocks to size. Affordability of any of this will depend on your supplier.