Are you using the space under both rear seats? You can fit an enclosure under the passenger side rear seat. There might be some room behind the seat as well. I'll see if I can find the video that shows how to flip down the rear seats.

2007-13 Toyota Tundra Double Cab Sub Box, Subwoofer Box

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netaudio.com

 

Here are some links for the back seat mod.

Rear seat fold down mod

This only works on Doublecabs. [ATTACH] Here's @JMB's original write up to cutting the hooks....

www.tundras.com

Fold DOWN rear seat in double cab mod

Hey all I randomly came across this video that shows you how to make the rear seats in a double cab fold down to take advantage of space behind them. Has anyone done it? It doesn't seem like a ton of room but certainly a functional amount of space for things I don't need immediate access to...

www.tundratalk.net