Put some spacers under the seat legs to get you a little more room. I put 3" spacers under my Silverado extended cab rear seat and built a box with four 10's facing forward. Don't know the architecture under your Tundra seat, but you don't have to live with the limited space. Jack the seat up. DO NOT put a box on your seat. It will become an 80-pound projectile.
If you want to see some pics of my under-seat box, drop me your email address and I'll get them out to you.