I hate being held hostage by manufacturers, especially when it comes to something that I can just as easily do myself. Even on the cheap, a barebones rod tube can cost $25, not including freight. Need a longer or thicker diameter tube? It'll cost you. It gets worse if you're trying to find a tube for a two handed rod. The Fly Shop sells a spiffy cap that you can install on a cut to length section of PVC pipe. It is sharp, but will still run you about $17 bones, plus shipping, and doesn't include the pipe. I can take that idea one better; why not build one yourself?
A visit to your local hardware or plumbing store will set you up with most everything you will need to build your own rod tube. Pick out a diameter of pipe that will fit the width of your rod sections in its rod sock. For most singlehanded rods, a 2" diameter pipe will suffice. For two handed rods and large saltwater rods with fighting butts, you will want to step up to a 2.5" or 3" diameter tube. For the sake of convenience and transportation logistics, five foot sections of pipe will be appropriate for constructing one rod tube. I opted to go heavy duty and selected Schedule 40 PVC pipe. PVC vacuum tubing or waste pipe will meet most of your needs for rod protection. ABS pipe is also an option. However, stepping up to Schedule 40 is a wise choice if your tube will be checked on as luggage on an airline, or if you plan on exposing it to the elements frequently or keeping it on a boat deck. The downside is that Schedule 40 weighs more.
Other supplies you will need are two female adapters, one cleanout plug, and one flush cleanout plug, all matching the diameter of your pipe. If you plan on solely using your case for storage and transport within your vehicle, you can get by with a standard industrial adhesive, suitable for PVC. I opted for Schedule 40 PVC, and will make use of PVC primer and plastic pipe cement I have on hand.
Start with measuring the longest section of your broken down rod. A good rule of thumb is to add 2" to the longest section. That will be the length you will need to cut your tube to. I used a miter saw to quickly and cleanly cut the ends of my tubes. A hacksaw or reciprocating saw (sawzall) will also do the trick. Deburr the edges of your tube, removing all plastic shavings and fragments. Clean the ends thoroughly. If you aren't planning on cementing the ends in place, you can apply your adhesive, press the female adapters on to each end, and wait for the adhesive to set, per the manufacturers instructions.
For Schedule 40 PVC, a plastic pipe cleaner is highly recommended if you are planning on using pipe cement. Clean the ends of pipe with cleaner then apply primer to the ends. Repeat process on the smooth interior side of the female adapters. Some primers are available as a primer/cleaner. Be careful when using the purple primer, as it will stain everything it comes in contact with. Apply the pipe cement to the female adapter and immediately press on the fitting on to one end of the pipe. Twist. Pipe cement will set up very rapidly so don't dilly-dally on this step. Repeat for the other adapter and pipe end.
For added insurance, I glued some leftover 6mm craft foam I had on hand to the inside ends of the cleanout plugs, in order to protect the rod ferules in case the tube gets jarred. I used craft foam adhesive to affix the foam to the PVC caps. All you need to do now, is screw in the cleanout plugs and you're all set. I used a Sharpie to label the exterior of the tube with the rod contents. Apply your favorite stickers and/or artwork to the tube and you'll be stylin'.
Total cost for one tube (not including the adhesive): $8.16 plus tax.
Total labor: 15 minutes
Knowing I didn't get ripped off: Priceless