Hi there Eric,
I have a whole bunch of sheets of the corrugated plastic you are talking about. You list that you are located in the Rockies. Are you in the Colorado front range area by any chance? I am in Ft Collins. The sheets are light, but expensive to ship because of there size (24"x36" or 24" by 18"). The rods can be most any metal that will keep its shape. I have aluminum and steel. I buy the wire by the coil. Steel is heavier but you need few rods for the same stiffness. Im not exactly sure where the break even point is.
To keep the wire from poking through the bottom, you do not actually need to add anything. The key is to cut the rods 1/2" - 3/4" shorter than the length of the plastic sheet. Then push the rod into the tunnel so that it is recessed about 1/4" on both ends. Then bend the sheet over the edge of a counter or over your thigh. The bends will keep the rods in place.
Ive used these types of frames quite a bit. Let me know if you have any other questions. If you are interested in the sheets you can email me at ron at r2packs dot com.