I decided to order a one-off custom drawer from Adventure Trailers for my LJ. The final dimensions are 34.5"x23.75"x9". In order to accommodate a drawer of this size I had to make three modifications to the Jeep. One, I moved the back seat forward 2" by drilling a new set of holes in the seat mounting brackets. In doing so, two inches had to be cut off the front of the brackets so they did not extend out into the foot well. Two, I removed the felt padding from behind the carpet over the wheel arches to gain more width. Three, I replaced the torx head bolts holding the seat belt anchors to the wheel arches with a hex head bolt without a shank. The torx bolts had a shank that kept the head from snugging up against the body, whereas the hex bolt can be snugged up tight against the anchor and the body. This prevents the anchor from rotating around the bolt but the extra clearance is worth the trade-off. With these changes I will have 1/4" clearance to the tail gate latch, 1/4" clearance to the back of the seat and 1/8" clearance between the sides of the box and the seat belt anchor bolts. The bolts are the limiting factor on width and the clearance to the tub along the sides of the drawer is a more generous 3/8". The height of the drawer was purely discretionary on my part. I wanted an available depth inside the drawer of 6.5". Martyn thought it would take about two weeks for delivery and I will post pictures of the install.
