See this:I haven't ever heard of a sub tub...
SUBTUB Gen 2 xB 08-14 D2BDST
Another option is the JL Audio Stealthbox, on the side:
SB-SC-XB/12W3v3 - Car Audio - Stealthbox® - Scion - JL Audio
As mentioned you can put a sub under the front passenger seat, but no one (that I know of) has put in a 12". The biggest I've read about is a 10".