[masraum]

I've had a few SUVs that had rails and bars on the roof that were OEM. For those, the rails were front to back and the bars could be slid forward/backward so you could get a decent distance between them.

What about this sort of thing? This is not our car, but it's almost identical (down to the color). 2018 Outback.
How would you strap something (or would you) like a sheet of plywood or a box spring to something like this? The bars are in a certain position as dictated by the installation instructions. I would hesitate to move them to the extreme ends since I assume there is a purpose behind the measurements made in the installation instructions. I am not that comfortable strapping something long to such a short base.