Few questions here....
I'd like to convert my bulky over-sized sub box that's taking up most of my cargo area to an under-back-seat sub box. I currently have two 12" infinity subs, I'd like to reuse, if possible. I know people put 10" subs under the rear seats and I can find pre-made boxes online for 8-10" subs, but having trouble finding ones for 12".
Will 12" subs fit under the rear seats, without sounding like garbage? And if so, does anyone know where I can get a pre-made box for two 12"? Or even a pre-made box for one under seat 12". Two is definitely over-kill, so I'm fine with only using one if it comes down to that.

 

It will depend on how much volume the subs require. Look that up, then do some measurements under the back seat to see if you can build the box big enough.

You will have to build the box to fit around the support legs. You will also need to figure out how to get it under there. I've seen boxes made like this, but it will take some skills. If I were building one, I'd mold fiberglass to the floor under the seats to maximize space, then frame 3/4" mdf to build out the rest of the box.

 

I made this box for 2 10 inch subs several years ago. Since my rear seats are only used by my kids, I increased the front angle to get a little more air space.....