Here is an idea, Build the box just like you had planed. make sure the base board is longer so you can mount it. Also make sure you build the box right!(clean cuts, pre-drill the holes for the screws, use wood glue and wood grabber screws. like the drywall ones.) The rubber will protect the pc from vibrations. It's a good idea to dynamat the truck too.
Here is a quick photoshop pic