Sometimes effective solutions aren't flashy, and this dual monitor stand certainly fits. It's essentially a pair of VESA mounts drilled into a piece of wood and mounted on legs. Is it beautiful? Not really. It is functional, easy to position on a desk, easy to build and cheap to make? Definitely. Instructables user crakensio put this together, and while you can see the front with his monitors mounted above, you can check out the rear view below. It could do with maybe a channel for cable management (like one of these baskets, for example) but it gets the job done, and everything you need -- from the wood to the brackets and the mounts -- are available at a hardware store or online for a few bucks. The only part that might be tricky are the legs, which crakensio says he got for free from an old machine, but anything that can attach firmly to the wood backing will work.
Even if you don't duplicate this specific one, or yours ends up a little more than $10, it's good inspiration for an affordable, portable multi-monitor mount that doesn't have to be a clamp arm mounted to your desk or something pricey that's difficult to move around.
DIY Monitor Stand for Less Than $10 [Instructables]