Because I'm terrible at WIPs, this is the last photo I took. But the rest of the steps are pretty simple.
First, I added a ring of foam/styrene to the top and bottom of the tube for added dimension.
Then, seeing as the bottom of my tube was still open, I made a little extension for the bottom. I took an oval of cardboard from the tub, glue a ring of foam/styrene around it, and then cut three strips of cardboard to glue on the inside and affix to the larger portion of the tube. (If you look at the finished, quiver, you'll see what I mean.)
After it was all together, I painted it with acrylic paint (inside and out), and finished it off with a few pieces of black craft foam for details. The arrows were taped into the quiver with electrical tape.
Check out my Hawkeye costume for finished shots. Hope this helps - good luck!!