I realize that this is "just plastic tubing". It's PVC. My original goal was to pick up a "real" rack from the LBS. However, I got there 4 minutes late and they wouldn't sell me anything. So, I stopped over at Lowe's and picked up $7.00 worth of PVC that I thought would be sturdy and lightweight. The slightly thicker tubing and the cement makes the whole system quite stable. I can lift the bike using the rack with no serious flex or strain.
I'm only carrying lightweight things, such as my schoolbag and a few odds and ends from the store. I think, after having made this, I'll build something a little more heavy-duty for my "beater" bike.

For the guy who made the wooden carry-all: I love it. I think you could probably shave a little weight by chopping down the fat parts. Either way, I do like it.