Depending on what you use to ground it with you can use a number of things. Silicon can be your friend when sealing things. If you use a bolt with a nut, teflon tape can help a lot as well. Put the tape on the threads of the bolt, and a small bead of silicon on the nut. If it's going to be permanent, I would have the nut on the outside and have a friend help you out with holding one end in place. A lock washer is essential so the vibration doesn't loosen the nut. I don't recommend using a screw in any way to ground the amp, it would be just a horrible connection and just cause all kinds of problems. With high-end amplifiers, you must, and I'll repeat, MUST have a PERFECT ground. The reason is the amp is in demand of a ground, and higher quality want it more than others. I have seen amps fry head units because the amp searched for a ground through the rca inputs. I haven't noticed anything about the cheap ones, but I'm sure it's just as likely to happen. Anything else, lemme know. I love audio.
