I saw a post on Reddit asking how to build a roof rack on a Fiat Ducato type of a vehicle, so here is how we build ours on a Fiat Ducato L3H2 2018.
When converting our van, we did not want to spend much on the roof rack, and we did not value our time either, so we designed and build our own. The hardest part was making of the roof rack mounting brackets. They could be bought for about 20-30€ a piece, but you can also build them yourself from U-shaped aluminum profile.
Finished mounting bracket 
One of the 8 mushroom shaped mounting points
Making one mounting bracket took me approximately 10-15 minutes after few hours of experimentation.
It is not necessary to drill these smaller holes. I think I used 7.5mm for all the holes and then cleaned the rest with a very rough file. Much quicker. 
Can’t remember if 9mm is bit too big drill size. I might have gone with 7.5mm in the end. 
I screwed the mounting bracket onto a wooden block and then cut it shorter with a jigsaw 
Approximate locations of the holes, from the side.
Take approximately 80mm piece of ~2.5cm sturdy U-shaped aluminum profile and drill 7.5mm-9mm holes into strategic locations. Make the profile a bit lower with a jigsaw, if needed. This piece should be then inserted into slightly larger sized U-profile (2.5-3.0cm, you can bend the sides of the larger profile to make the fit looser). Then trill two holes through both of the profiles and insert two low profile bolts through the holes. Do not make the holes too big, as then the bolt will turn loosely and you can not tighten the bracket. Note that if inner profile is too high, you can not tighten the bracket on the roof as the inner part will hits the surrounding piece.
We added rubber sheet under the bracket to make sure that the roof is not damaged.
The roof rack itself is made from 40x40mm profile from https://easy-systemprofile.de. There are many other alternative sources to use in Europe, but for us they seemed like a good compromise. For easier installation and cheaper postage we decided to cut the long profiles into two shorter sections and connect them with connectors. The profile lengths are 2x1900mm, 2x1600mm and 5x1500mm. 1500mm profiles could be approximately 10mm longer, as in our case the profiles are few millimeters too short and there is a small gab between the connections.
The roof rack is bit too low (it can be lifted bit higher than in this image). Adding 1cm more height would make installing the solar panels easiers. 
The end results is pretty sturdy and low profile and cost bit less than 300€ with all postages, nuts and bolts.
