There are commercially available L-brackets for most cameras, including the D300, but they use an expensive type of quick-disconnect called an "ARCA-Swiss". Not only is it expensive, but it's big and requires the photographer to twist a clamp to affix the camera.

Another choice is the Bogen-Manfrotto QD, which is smaller and works differently...you just put it in place and a spring-loaded clamp snaps closed...it's quicker! I like this approach, but couldn't find any L-brackets.

Thus, I bought two Bogen-Manfrotto QD plates and fabricated my own. It's made of multiple Aluminum pieces that are screwed and glued together, along with the QD plates. Then I painted it. The pin fits into an existing hole on the bottom of the D300 and keeps the L-bracket from rotating.

Here are pictures of the completed project: