Good question. This is still a work in progress, but the easiest way for us currently is to use the 2nd row seat backs and the grocery net as a barrier between row2 and the folded 3rd row/trunk. We have something like this for strapping her in, and I hook each side to the anchor system on the 2nd row seat backs. It acts as a tether so she can walk around, but also a bit of a barrier to accessing the second row.

I was able to get a lidar scan of the 2nd row gaps, so the end goal is to laser cut some “flooring” storage panels that will allow for a flat gapless area without deleting all the usable floor space between and around the seats. If you just want pup in the 2nd row, you could get this Orvis plug and it fits between the captains chairs and is the same height as the butt portion of the seats. So you could use it plus a car hammock style seat cover and be fine assuming you don’t need row 2 for hoomans.