velcro is fine for the testing phase, but it certainly won't hold it in an accident or prevent thieves from easily removing it. always use brackets and screws to keep the gear secure.
don't use drywall screws in a car, ever. drywall screws are for drywall. Use construction grade screws, tamper resistant screws, etc. self tapping screws are for securing brackets to the metal in your car. i usually try to use square bit or torx bit as well.
i prefer to mount amps to a separate carpeted board, then secure the board to the vehicle so it is out of the way on a side. that way i can easily move the sub box around, or remove it completely, as needed. some will mount the amp to the back of the rear seat, but still use a board. you don't want to mount an amp directly to car metal.
sub positioning will determine performance. the best place to start is the one with the least amount of destructive phase interference. usually, this is in the rear corner of the trunk, facing back or to the side. don't adjust it until it's loudest, adjust the position until you get the most balanced, dynamic, and tight sound.
