Another option for you guys to think about is to cut yourself a piece of steel (or plastic for that matter) large enough to cover up the holes. From there you can paint it the same color as the truck. From there you can put double sided tape on the back side. From here, you can get your hands on some "clear decal paper". You create your lettering however you want, print it on to the decal paper and once the ink is dry, spray some clear coat over the printed letters. From there, you can cut out the lettering from the decal sheet. At this point, you drop the lettering into a bowl of water to release the clear sheeting from the paper back. Slide the clear film off of the paper on to the painted steel/plastic. Center and then blot dry with a paper towel to make it so the decal will not move. At this point, you can use either "Solvaset" or some 99% rubbing alcohol and apply a little bit around the edges to seal the decal to the surface and this should also push a lot of the air bubbles out too. It is especially useful if you only apply the Solvaset on one side and let it run down between the decal and the surface. Let dry and then apply a final layer of clear over the decal.
At this point you are ready to mount everything to the back of the truck. This works especially good on white trucks. But can be done with any color truck. Just keep in mind that you will not have any white lettering there unless you have a special printer that prints truely white in (very few printers do and those that do are very pricey). Also, the darker colors will tend to make the printed lettering a bit darker if you are trying to print a light color letter.
If you need some help, let me know. It is fairly easy to do once you get a hang of things.