Sorry, I'd almost forgotten about this. Regarding clearance for items stored on the shelf, take a look at the contour of the roof line in the cargo area. There's more "head room" on the interior though the space slopes toward the rear and sides of the vehicle. The lines of the Trekboxx cargo system take this into account so you can roughly use the cargo barrier as a guideline though it also leaves a bit of clearance between itself and the headliner.
The ruler is slanted a bit in the photos so I'd reduce the measurement a bit. The rear-most section of the shelf has about 8.75 inches between the headliner and the shelf. That's with the 60L NL fridge... the 60L ARB fridge has much more room since it's a bit wider (and maybe longer). The forward part of the shelf has a little over 12 inches of clearance between the shelf and the headliner. The headliner slope is gradual as it's about 13 inches from the rear of the shelf to the point where you get full access to the 12 inches of height. You'll also notice that the contour on the side of the reduces the height to about 7 inches.
The Yeti 1000 would fit, technically, but it'd be difficult to access / remove as you'd need to install it first then mount the shelf. If I may, I'd consider putting 2 of the Lithium 400s up there if that would work for your needs? They'd fit and they're just 16lb each.
Here are some photos to help...
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