If I were you I'd build a little shelf out of wood to support the front of the amp. Take some cardboard an cut it to match the lines of your floor (for the sides of the shelf), then make the proper shaped piece out of wood. Then, one square piece on the top, one on the front, and carpet it. You can use self tapping screws on the rear of the amp (which I assume will be screwed through the metal truck floor) and some wood screws towards the front. I think that would be pretty trick.
Also, you should think about using some sound deadener, it works.