if u put your woofer like the first pic there, u will have a hell lots of vibration!  the floor board will be shaking like earth quake and your metal body will vibrate too. except if u put thick mat like the superlon under your woofer to reduce the vibration. but its not 100% sealed so its not that perfect interm of performance.

i would recommend  ditching the woofer and get a real good component set and test it out first. some quality component set would have enough bass that your friend tot you are running a 8" sub. or even a 10" sub.
everything need to blend in, like your amp, your crossover and component... try the focal utopia or K2 series..  if u have the budget.

if ur purpose of getting a sub is for blasting (sound pressure level SPL) then you definately need a big enclosure or a very good free air type of sub. all boil down to your wallet....