Very late to the party on this thread...
For mounting my rear/side cameras screen (about the size of a small tablet) I use one of these:
https://amzn.to/46DWdi2 Lamicall Magnetic Car Phone Holder Vent 2023 Upgrade 6X N52 Strong Magnets, Universal Mobile Phone Mount Hook, 360 Rotating Air Vent Cradle.
I've found that I can still open/close the vent by mounting it at the top right corner.
For my phone I use this: (the vent attachment not the suction cup)
https://amzn.to/3QqcKAs VANMASS 2023 Ultimate Car Phone Holder [BEST Suction & Clip] Rotatable Universal Mobile Phone Mount Accessories Automobile Cradle for Dashboard Windscreen Vent Van
I'll see if I can get a photo up soon.
In the past I would suction cup stuff to the windscreen but I rented the van out and one of the customers was a lorry driver who'd also been an AA Inspector in his time. He loved the van but told me after: "That rear view screen there (was above the dash) is an instant MOT fail and because it's blocking the corner of your windscreen (Driver's side) then if someone hit a kid or something you might find yourself in a load of trouble". Which is why I now have it off to the side between door and steering wheel, not blocking any line of sight of the windscreen. I know a lot of people put their phones above the height of the dashboard, but I wonder how "legal" it actually is. I know there's a lot of hoohaa about where to mount a dashcam/crashcam.... I'm no legal expert but I've played around with different positions for my phone, tablet and camera monitor and now I've got all my stuff out from line-of-sight of the windscreen I'm a lot happier