I thought I'd do a little write-up on my homebrew version of the rear storage tub enlargement. Overall cost for this mod was very low at roughly $65, which was spent on the new muffler and a couple of clamps, everything else I had laying around. Even if you were to buy the box and some metal to attach it with, you'd probably be spending less than the aftermarket tub options currently on the market. If I were to do it again, however, I'd probably buy a sheet of steel and build the box to fit better, rather than squeeze one of these 81mm mortar shell boxes in there.
First off, you'll have to remove the factory muffler and heat shield (obviously lol) and toss them in the scrap pile. Removing the heat shield is pretty straight forward, 4 11mm nuts, I believe, and it'll come right off.
Next I marked out my cuts. If I were to do this again I would not go so far out on the sides of the factory tub, you'll see why in a minute.
The next step is to cut a gaping hole in your Jeep. The first cut is the hardest. I'll admit I was pretty nervous to cut into a perfectly good Jeep but I knew in the end it would be worth it.
Test fit of the 81mm mortar shell box. Here you can see why not to cut so far from the edge of the factory tub because now all the open area needs to be covered back up and sealed. Oh well, we learn from our mistakes, right?
Recycling some sheet metal I had laying around from I don't know what, I riveted it to the box then laid down some weatherstripping and RTV to seal out the elements and any potential exhaust gasses, then riveted the whole shebang to the floor.
I had originally cut the section out that contained the factory cover latch thingy and decided that since I would be using the factory cover for the time being that I'd also like to reuse the latch as well, so I welded that section back in after modifying the latch to work in the slightly altered location. I covered the exposed metal edges with door edge trim to avoid cuts and scratches. After it was all done, I sprayed it with Duplicolor Truck Bedliner.
And the final step was to install the new muffler. I chose the Dynomax 17230 Ultra Flo Welded muffler similar to the one used in their systems for our Jeeps. Its just barely louder than stock, but has a slightly deeper tone to it.
The last picture shows the stuff I keep in my Jeep laid out and packed away in the new space.