Deflectors on roof DO work.

if set forward a bit an bultto suit.height, angle and width.

why do you think ALL the truckies mainly use them.
LESS fuel used. Plus a big box of cr-p behind it too chuckle.
Look up dimensions on net for wind deflection and start sawing.

With vans though, it's not normally worth the hassle or cost,
for what you'll get.
Aerodynamics are pretty boxy on both

Mate has a Troopy with roof rack and a big offroad van.
He has a nose deflector on bonnet. leading to one on leading edge of roof rack,
and one rising from rear of that to cover front of van.
YEP. It works fine he says.

But when we travelled together. (Yeppoon to Airlie most yrs)
Me with a patrol GU and 6.5mtr Offroad Roadstar. (3 tonish).
I noticed he ALWAYS used a lot more juice than I. Same roads. Same speeds.

But I am a very frugral driver. 9.5ish kmpl driving, and 6.2\6.5 kmpl.
Towing at road speeds mainly. (with that rig over 14 ish yrs)
She was a beauty.