• PVC tubing -- we bought 12' of 1/2" pipe from Home Depot (about $5 total)
• Packet of 1/2" PVC T-connectors (about $2 at Home Depot)
• Packet of 1/2" PVC 90 degree connectors (about $2 at Home Depot)
• PVC pipe cutter
• PVC primer/glue
• Tape measure
• Sharpie or other kind of marker
• Spray paint, if you want your rack anything other than PVC pipe colored
• Bungee cords (24" seem to work well)

background.
Peter and I are going on a dual-sport ride with Sport-Touring.net next weekend, but the rack I bought is backordered and won't get to us in time. I didn't want to spend a lot of money or time hunting down a different rack, so we decided to make these temporary el-cheapo platforms to strap our bags to.

procedure.
The "hardest" thing about this project is figuring out what you want your rack to look like. How wide? How long? One cross-bar or two? Peter and I tried a few different configurations (hence buying the extra PVC pipe) before settling on the following:

• Four 5" long pieces
• Three 12" long pieces
• Two T-connectors
• Four 90-degree corners

This gave us a nice square platform with one cross-bar, measuring about 12" by 14".

That said, this is completely personal preference and also depends on the size of the stuff you're carrying. If you're carrying a tent and gas cannister, you'll want a larger platform. You get the idea.

Once you figure out what size you want, it's time to measure and cut.

Use the measuring tape and Sharpie marker to mark your pipe lengths on the PVC. 1/2" PVC is easily cut with the ratcheting PVC cutter that we had -- anything larger and we may have wanted to use a hacksaw instead.

Once we had everything cut, we assembled the rack without glue to make sure the size and shape was what we wanted.

We experimented with different bungee configurations, too, to ensure that we could actually keep the rack solidly attached to the bike. Best to do all of this prior to gluing, in case it turns out that your first idea won't work.

Now it's time to glue! I made a huge mess of myself. Fun! First, apply the purple primer to the outside of every pipe and to the inside of every connector.

When you're done with the primer, line everything back up the way you want it. The glue will dry really quickly once you apply it, so you want to make sure that you know which pieces you're going to glue together.

Now it's time to glue. Do one pipe/connector at a time, and remember that the glue dries quickly. Just smear a good amount of glue on the outside of the pipe and the inside of the connector, connect them (squishing glue out all over your hands -- it'll peel off when dry) and let dry.

Now you will have this horribly ugly looking rack!

Peter and I chose to spray paint our racks black. We just used normal old rattle-can spray paint. Remember to let the paint dry completely before putting it on your bike.

result.
Not too bad-looking! We'll see how they hold up to a 500-mile weekend. ;)

update.
The racks held up really well on the 500-mile trip -- totally rock solid. I was honestly surprised at how well they ended up working. Not a single slip or slide, either on the freeway or going over rocks and ruts.

I did end up replacing the PVC racks with "real" racks, but that's only because apparently I like spending money. The homemade racks would have completely held out for a long time.