I read a journal on CGOAB where they carried spare tires. They chose Panaracer Pasela Tourguard tires because they come folded in a small tight package when you buy them. I haven't personally used those tires (I have Marathons), but the Tourguards can be found in a wide variety of sizes and are pretty inexpensive ($22.99 here). I found an image that shows them rolled up in the package they are sold in: