so why not use something that has an alternate function or two? You ask, what?!
I have two waffle board/bridging ladders mounted on the roof sitting right on the factory rails. I stand on these for photography but can pull them off from the side in under 4 minutes to use for a number of other purposes such as:
1-their intended uses: bridging a gap, ramping up a ledge, traction in sand/ snow, mud
2-blocking under a wheel for leveling during camping
3-showering floor if camping and staying a while these make a great shower floor in grass, dirt, etc
4-flat surface with towel over it works as a camping table or prep surface on the ground or elevated
5-bottle jack base in sand or other soft surface where you need more height for tire change
-etc
Mine are mounted using my prototype test parts that just kept on working so i never refined them. ingredients list:
1"x1/4" flat stock aluminum stock
1/4x20 threaded rod
wing nut, washer
file, tap, hack saw
I made my own little drop-in tapped blocks like a roof rack might use but not rusting and unable to come out accidentally. I cut the flat stock about 1" tabs, tapped center, file edges to allow easy drop in at the wider part of the factory rail. Cut the threaded rod to rise above the 2" waffle board just enough to start and tighten down the wing nut on a lock washer. Stainless rod would be ideal and stainless nut.
Procedure when all parts ready:
-drop 1" aluminum tabs into rails at wide section
-slide to intended restricted section of rail to stay secured
-place waffle board up onto rail, VERY carefully slide over far enough on glass roof to allow reaching from other side (this requires patience and care. you could first put a light piece of wood like a 1x6 across to use as a bridge-ramp until you get hang of it, then remove from other side of course)
-reach in a bring rest of waffle board through and set onto rail. (these are 48" long boards so they are perfect for the roughly 44" rail to rail width on roof)
-thread rod into the plate down in the rail (yes, you have to know where it is. easy if nothing on the roof and you can just stand on the tire to look through the waffle board to line it up)
-thread on the wing nuts.
-reverse for removal
These boards DO flex a little but mine have never touched the glass roof. I weigh no less than 160, more like 172 fully dressed in normal clothes. I can stand in the middle but obviously if you stand more toward either 1/3 from the middle, then you dramatically reduce change of flexing into the glass. Normal stance to feel stable when photographing, my feet are not in the middle anyway. You could easily allow for lots of flex by having some sort of spacer between the rail and mounted bridging ladder.
I already had these fiberglass waffle boards but imagine there would be almost zero flex standing on a true Mantec bridging ladder, not the sand ladders that flex but the beefy ones with raised sides. OKoffroad sells them I think.
This whole system also fits UNDER my Hannibal rack but I often don't need the rack.
Good luck.