HiYa…..I also have a Transit...….I used large residential panels......66" long that span the van width
The Transit has roof mounting points already factory installed...…..with a metric nut captured/welded to the roof rafter (under plastic plugs)
I made a "pad"...…..3"x5"x1" that was tapered about 7 degrees from some solid PVC used for siding and trim in construction.....This makes the mounting surface LEVEL
Then I ran long 2" L-shaped aluminum extrusions front to back ….attached to each mount...….(w/sealant)…...
I then attached more sections of the same angle to the bottom of each of the panels...…sized to interlock with the roof rack
One UP......One DOWN...….I also used EYEBOLTS to join the two angle extrusions......Perfect attachments for a DIY awning/tarp later