What sort of thing are you carrying? I would love to find a lightweight alternative, I have fitted closed cell foam to the top sections, but I only carry fishing tackle so no worry of large bulky items bashing against the sides.
I like the standard lightweight plastic lower items as fitted as standard, I know Nissan sell original upgrade items for the higher sections.
I'm a floor layer by trade so manly tools and the occasional bit of flooring also for my fishing gear as well I'm going to build some racking on one side so I can set up my bed chair as we'll for my cheeky over nighters mid week
If it were me, I would get some cheap hardboard and make your own patterns up from that. There arent any particularly difficult shapes to cut.
Fortunately, mine came pre-boarded out. It has a panel for each door and one for each side, so 6 in total. Attatched to vehicle with selftapping screws. Pretty straight forward. The side panels are slightly more complicated because of the wheelarches. Personally, I would make and fit the panels with cutout for the arches, then make the arch covers afterward (mine are glued on with silicon, and have a single selftapper through the wheelarch).