another way
i've seen this done and have done it with a few guys who were too cheap to buy the adapters. but you'll have to take the interior back cover off to expose the metal, run a self tapping nut (short) just long enough to get a good bite into the metal or drill the hole and use nuts and bolts. as i said i've done it and regardless you'll have to take off the cover to install the adapters if my memory is still intact. walmart sells the adapters for about 20 maybe alittle less, don't pay more if you do walk out and go somewhere else.