If you flush mount it, that will be fine. I just don't think you should get them any closer to the roof of the trunk. Even another 3/4". You need as much clearance between the subs and the roof of the trunk as you can get so the pressure can get into the cabin. It's already lagging behind the port. No need to trap it any further.
It is sometimes beneficial to have them loading off of a wall, but I don't think that applies in this case.
yeah i was thinking about doing thatCan you do threaded rod?