You should be able to use the Mold tools to do this direclty.
Right click on any tab to load the Mold Tools.
There is a Tutorial to show how to create a simple mold.
If you have trouble figuring it out - attach your sldprt file here.
I attached an example file.
Note that there are 3 solid bodies in the file.
Your problem will be slightly different in that you don't have a planar split line.
Ice Cube Tray.SLDPRT.zip 1.2 MB
You might be able to use the Replace Face command. Sketch a rectangle that is 12"x36". Use Extrude Surface to make a body with four sides that is 5" deep. If you have four faces on the sides of your skateboard model, you should be able to Replace each Face by one of the sides of the Surface Body. You might have to use Split Line to make four faces if you only have one or two.
Another possibility is to go back into the Feature Manager for your skateboard and change the way you build it. Make it as a Sweep or Extrude that is 12"x36" in plan view and then cut it to the required shape in the plan view.