Here's a pic of the Pro.Fit antenna mount plate that sandwiches behind your license plate, either front or back:



http://www.pro-fit-intl.com/New_Products/

Once you look at it for a bit, all you really need is some sheet metal, some snips, a vise and a drill and you're good to go. For the VX rear, I'd cut a piece that would look like a license plate with a triangle on top. You'd bend a 90 where the triangle meets the rectangle and put the antenna toward the point so as give decent clearance. Relieve the edges of the aluminum plate to keep it smooth and you can take it off anytime.