Do you own an SPL meter or have easy access to one? If you do not you will never be able to find gains.
What vehicle? What sub? What amp? Post pics of the box as it sits.
There's a lot of tricks that gain gain but you absolutely 100% MUST use an SPL meter to test and keep notes. This thread over to SMD forum is a good read, bear in mind just about everything cheap (moving visors, opening glove box, adding some gorilla tape) is mentioned here and not all of it gains for the OP. You must try every variable and every combination of variables, for example scrunching your passenger seat all the way forward may gain a couple tenths now but after you find a couple more tenths from repositioning the box the same seat move may lose a bit.
Results so far!!Gain:- Moving box around the trunk until i found the sweet spot: 0.5 dbs- Changing box size until i found the best one: 2.2 dbs- Removing sub´s boot: 0.1 dbs- Negative wire run from battery to amp instead of grounding it to the car: 0.3 dbs- 2 extra batteries: 0.2 dbs (depend on t...