Both on our Sprinter and 2020 Transit, we used a
basic cheap-o Amazon solar entry gland. I used VHB tape and nothing else to hold them on and make them water-proof. The Sprinter never leaked in the five-ish years we had it. The Transit hasn't leaked in the three years or so that it's been there. The through-hole is in a location that I can see from inside when doing wiring work and there's no evidence of leakage.
Here's the Sprinter gland from when I pulled the flexible panels and replaced them with rigid panels.
When we added the external cellular antenna - and later, the Starlink - I used a
cheap water-proof box to handle all those cables plus more for lights in the future. I added a few different sized through-fittings to accommodate current and anticipated needs. I cut a similarly-sized hole in the bottom of the box and in the roof of the van and VHB'd it to the roof. No leaks in a year-and-a-half. Just the cellular antenna here; Starlink ethernet/power cable runs through it now as well.
Doing it again, I'd probably just do something like one of
these or
these and I'm sure it'd be fine.
FWIW, I'd rather cut extra holes than use the ones that are just openings from the factory. I'm using the rear-most factory holes to increase the strength of the roof rack. The extra ones that are still up there are a weird inset; I have more confidence in cutting my own holes on a flat surface.
