You could powdercoat/anodize/polish the tank and place it where the opening is
The sub box should be whatever JL audio recommends on their website:
http://www.jlaudio.com/10w7ae-3-car-...-drivers-92114
Wall Thickness 0.75 in / 19 mm
Front Baffle Thickness 1.0 in / 25 mm
Volume (net int.) 1.25 cu ft / 35.4 L
External Width (W) 14.5 in / 368 mm
External Height (H) 14.5 in / 368 mm
External Depth (D) 15.375 in / 391 mm
F3 40.5 Hz
Fc 43.5 Hz
Qtc 0.764