I got my cyan Lumia 900 yesterday, and realized that there aren’t any docks available yet — so I built one myself using Lego. Similar concept I used with the Lego dock for Samsung Focus last year.
Below is the dock without the phone. Back wall is tuned very carefully. Because the phone needs to slide in from right, and has to align “perfectly” in order for smooth docking operation. I’d say tolerance is less than 1/10th of a millimeter. This will take some time but be patient and try to match what I have done exactly. Every layer has a purpose and allows phone to rest reliably and slide in perfectly. In addition, undock process has to work one-handed. For that reason, you would want to keep the top of the wall holding the cable end as low as possible. This allows for your thumb to push the phone out from left without having to hold the dock with your left hand.