Ok good, keep in mind that anything you do, if you compete, may be illegal so check rules before competing as some gains may not be legal.
Very basic- whatever side the meter is on, need to find out which side is louder in the vehicle.
Typically, smaller vehicles are louder on opposite side of car vs where enclosure is.
Longer vehicles, it's the same side. Need to test SEALED to find out.
Once you know, The side it's loudest, keep that window rolled up, and roll the opposite window down.
Even opening the door on the side with window down can make it even louder.
Test other things like sunroof and other doors and windows to their effects.
The % of opening will vary based on frequency.
Remove carpet and soft fabric out of vehicle completely.
Even seats need no fabric above them. Custom seat covers in few smooth finish will gain.
Displace space in cabin to compress air volume.
IF you are not running a burp only setup, locate resonance of vehicle and burp at that note.
Run amplifier at maximum supported DC voltage by using custom battery banks you can build yourself or add individual cells to.
Remove headliner and reinforce roof with steel, Anti-Drumming mat, wood, etc..
Do the same to your doors as well.
Reinforce roof at windshield to prevent flex.
Install custom thick glass if running high SPL (160s+)
Use Concrete ANYWHERE you can.
Even if using a single sub street box. Build another box filled with concrete on all 6 sides.
If this is too confusing for you, replace concrete method with Marble or Granite but will cost much more.
Later on down the road-
Use more Anti-drumming or Deadener to alter vehicle Resonance to a range that's more efficient for the vehicle to alter cabin gain.
Make everything in vehicle super-smooth. Glass it, line it in polished metal, something..
Use high quantity and large cables when capable. You want 0 voltage drop, least resistance as possible.