Not trying to argue, but a capacitor is useful for something such as a long bass note, but otherwise not too useful (mainly just more wiring, but do have a
volt meter).The battery then has to recharge the cap before giving power to the amp. Adding a second battery will increase the amount of time you can listen to the audio system with the engine off(party party

). When the engine is running, the secondary battery is an additional "load" for the alternator to charge. So an upgrade to a larger alternator should be considered.
-As stated above your best bet is to pick up some MDF wood, measure it, and hope you took wood shop in highschool...
