I don't know much about restraining it (although a board with some bungy cords might work. As to interfacing the audio with your current sound system, You can go to almost any place that sells portable CD players and get a "tape adapter". It is a unit which looks like an audio tape, but has a cord coming out of one side. You slide the tape into the tape deck in your van and the cord gets plugged into the headphone/speaker jack on your laptop. That will get your MP3s playing through your car audio system.

Sean B.
'90 V'gon, "Happy Bus"