I'm working on the gutter mounts for the rack extension. I bought an inexpensive gutter-mount van ladder rack (
https://www.amazon.com/MaxxHaul-70440-montaje-canaleta-capacidad/dp/B019DP6UG0) with the idea that I could modify the gutter mounts and use just them and as a fallback I could use the gutter mounts plus the included crossbar:
It worked out that modifying the gutter mounts is all that was needed. The rack is about 2" narrower than the gutters, so I modified the gutter mounts to jog in 1" on each side.
In these next photos the mount in the foreground is modified and the straight one in the back is unmodified. The modified one is still taller than necessary so it'll be trimmed shorter in the next step when I connect it to the rack extension. When the main rack basket is off the Jeep, it hangs from the garage door rails several inches above the Jeep - it's hanging in these photos so it'll be several inches lower when it's on the Jeep.
The unmodified mount is leaning inwards a bit in these photos, with the rack lowered onto the Jeep it would lean even more. I'm sizing the mounts to the main rack basket in these photos because it's sitting above the Jeep right now but the extension is the same width so the size will be the same.
Tip for bending something like this: these are made from 1/4" steel, which would be tough to bend nicely, so I used a cutoff wheel in my angle grinder to make a groove at each bend location. The groove is slightly deeper than halfway through the metal, leaving a fairly thin bit of metal to bend. Once bent, the groove is welded up to restore the strength. The grooves are made on the outside of the bend so when bent the groove will open up, making it easy to lay a nice fillet of weld in the groove and get great penetration.
While I had the welder set up, I replaced the 6mm nuts welded to the brackets for the clamp bolts with 1/4" nuts. The Surco rack adapters I use to mount the main rack to the Jeeps secure with 1/4" bolts so by making this change only one size wrench is required to secure everything.
The JKU roof is a bit wider and slightly lower than the LJ roof but I believe I can make these same mounts work on that Jeep too, but if that doesn't work I still have the second set that I can modify.