You can do better than that with the D300.
You can take multiple shots with your camera on the same image file.
In the manual mode, just turn the shutter speed wheel until you see 30" and you will be set on a timed exposure of 30 second .. one click further you can shoot in the bulb mode ... however long you have the shutter depressed, the shutter will stay open. I use a remote shutter release that can be locked down to I can take three or four hour long exposure of the night sky and get great star trails. The remote for the D300 is the MC 30.
Multiple exposures are discussed on page 186 in your users manual
Be sure to use a tripod.