I bought an aluminum Gr8tops Secure Trunk Box a year or so ago but had never bolted it in. Partly it was a test because I was also very interested in the Tuffy Security Deck. Bolting the trunk in is safer in the event of an accident. It makes it harder to steal. And it keeps the trunk from moving around when driving over bumpy stuff.
Bolting it in was fairly simple. I have a 1" body lift, which helped. Otherwise you'd probably have to drop the gas tank to get a wrench on the nuts under the tub.
Step one, drill four holes in the tub. There are two sheet metal cross members spot welded across the tub. I measured very carefully from a bumper bar that was accessible from both above and below. So I measured underneath to decide where I wanted the holes. I measured from the top to locate the holes. I had to drill from the top because I was too lazy to drop the gas tank.
Step two, is to punch some holes through the carpet and pad with an awl and open them up with a drill.
Step three was to measure the holes very carefully including both diagonals. Measuring the diagonals is important to avoid getting some funky parallelogram. If the gas tank were out, you would just mark the holes from the underside with a punch or marker.
Step four was to drill the holes in the trunk. Mine are 5/16" and the machine screws 5/16x1.5". I did a test fit after each hole. Here is a picture of the machine screws installed.
Step 5 is to install the trunk with machine screws from the top and a nut on the bottom. I planned to use a lock-washer but was only able to get the nut on with the limited space. Because the carpet and pad are thick, when compressed they keep some preload on the screw. I'll have to revisit this some day, maybe adding a lock nut.
Here is the trunk with its usual load: a hefty tool box, an air compressor, a milk crate with some recovery gear, some recovery straps that aren't visible, and a Cobra 75 WX ST radio handset. On top of this there's some room for daily storage, like a few daypacks. In the long run we plan to install a hard top so that the whole Jeep is more secure.
Here is the picture of the installed trunk with the top closed.
This fits behind the rear seat of an LJ. For trips we take out the rear seat to have more room for gear. We'll probably sell the rear seat in another year or so as we haven't used it yet.