Disclaimer: I'm not a doctor, I suggest you read up on some ergonomics basics, e.g. at daskeyboard.com. This has worked for me in half a year usage now, but only in combination with frequent mini-breaks, stretches and posture shifting by alternating between a sitting and standing desk.
Polystyrene can be purchased in any home improvement store for a couple of Euros, it typically comes in 1m+ sheets and is used for insulation. You'll want to get the closed-cell stuff, which is much easier to handle than open-cell styrofoam (very messy). The shown holder is built out of three sheets with 2.5cm thickness. In general, thinner is better, since its easier to layer than to cut away large parts.
I've used an Apple Wireless Keyboard and Trackpad combo, which is best for free movement - but in general, should work just as well with cables and a mouse.
For cutting, a simple craft knife should be strong enough. The different layers can be held together by long nails, or wood screws for more rigidity. You'll want to get your keyboard on a slight negative angle facing away from you, so your wrists are in a straight line with your fore arms. The ease of working with polystyrene makes it ideal for incremental tweaks - just try an angle for a day, then cut away a bit more if you're not comfortable yet. I've also angled my trackpad sideways to accommodate a more natural, less twisted arm position.
Finally, polystyrene isn't the prettiest material. If you have a spare piece of fabric, cut it to size and secure it with a stapler or thin dressmaker pins.
Read more about the DIY standing desk on my blog, or download the desk model including a mockup of the keyboard holder into Google Sketchup.