Here is the start of the frame. I went with 1x10's instead of 2x8s simply for weight savings.

 

I used a marine grade carpet that has no backing, it resembles felt and is easy to manipulate. Like for speaker boxes, just higher quality. Glued on with all-purpose flooring adhesive.
Lowes and Home Depot should carry something similar, sometimes in rolls that are 3' or 4' wide. Or a remnant from a carpet store.

As for the gutter mounts for roof rack, I cut down the mounts by about 3". Not sure why they are so tall.

 

Ill have to look in to it. The mounts are tall enough to let the rack sit over the stock roof rack cross rails. How much did you cut out of those mounts? just curious

 

Cargo box/ dog deck is done. Just needs carpet.

 

Looks great!! I need to get back over to Wyoming for a goat hunt.

 

where are you from? We have t9ns of goats lol.

Went out chasing some pheasants

 

Currently residing in Bismarck, ND. Went out looking for pheasants today, it was a bust.

 

A bust on ditch parrots in ND, is that even plausible? Haha

 

I cut off about 3" of the gutter mounts. Basically I set the rack on the stock rails, on rubber bumpers. Then attached gutter mounts at or near pillar supports. Due to the domed roof of XJ, the crossbars of roofrack sit just under 1" above roof at center.
The attached picture shows the mounts redrilled, before cutting off excess. Also made a prototype wind fairing for it.

Cargo Box looks effective, is it secured to floor? That spare tire mount can be bent over if you prefer.

 

I am working on a securing method. Might just go for large self tapping crews and L Brackets. I trimmed the deck to fit over the tire mount tab. I need to drop my rack, it sits way to high for my liking.

 

But if the pup gets knocked into that tab, I imagine it would hurt. I bent mine over.
If you use screws, I would insert them from underneath the jeep with a fender washer, just inside wheelwell and straight into cargo box. One on each side, then remove them, spray hole with undercoating, reinsert screws and spay again. I also used the factory tie-down as two attachment points.

 

I put the "frame rails" of the box flush with the wheel wells, so i could easily do that one. My other dilemna, is i have one of those hard durable plastic liners that fits the back of the seat and continues down on to the floor and is custom molded. I dont really want to drill a hole through that.

 

Another option is eye-bolts in cargo area, then strap across box to eye-bolts. I have six heavy duty eyes in the cargo area. They are attached to interior sheetmetal where metal is doubled or tripled, fender washers on both sides. Only two eye-bolts are on the floor, the rest are elevated.
edit; added pictures.

 

Hmm. That might be the avenue i go

 

Weight Test on the Cargo Box, It works!! 110lb Lab, 60 lb Lab 45 lb Lab