I would think the best way to install the woofer would be to lay the cabinet on it's back carefully, (to not damage any amp controls), taking the weight off the screws for mounting. Maybe easier in theory though, 'cause you need to have room to turn the sub over.
If you are looking to lay the sub on it's back to install the woofer, I would use some hockey pucks or similar object,ie. wood blocks, around the perimeter of the back to keep the amp switches and knobs from ever coming in contact with the floor.
