I have a box for two twelves that's made to fit behind the seats in the standard cabs but you have to loose all of the storage bins

 

I'm looking to put 2 10s under my backseat in my Ram too sometime.

I know there are several companies that sell boxes on ebay that are right around $100 that are supposed to fit under the backseat.

Anyone have any experience with them? I'm trying to find a box and that's the only place i found them at that'll fit under my seat.

 

$100 seems to be the cheapest box for under rear seat. If you want cheaper, build your own out of 3/4" MDF and wrap it in your favorite carpet.