Has anyone seen or used one of these ?

https://www.cdslotmount.com/

It looks very simple but effective

Yes. My friend invented it. Very good  FL2 HSE Lux 2011 SD4 Gallway Green.
Fiesta 2014 Ttanium x 1.0 ecoboost - Fantastic 125 bhp 1.0l 3 cylinder turbo!
1976 Triumph Spitfire. Owned for 25 years Fully restored, magazine features - not quite standard
1985 Toyota MR2 MK1a. The oldest surviving example in the UK and one of only 5 left in 8B8 Blue

seems like a damn good way of completing ruining your cd player to me!

It doesn't damage it...although I personally wouldnt use it with a loaded in dash disc changer like the fl FL2 HSE Lux 2011 SD4 Gallway Green.
Fiesta 2014 Ttanium x 1.0 ecoboost - Fantastic 125 bhp 1.0l 3 cylinder turbo!
1976 Triumph Spitfire. Owned for 25 years Fully restored, magazine features - not quite standard
1985 Toyota MR2 MK1a. The oldest surviving example in the UK and one of only 5 left in 8B8 Blue

I would worry it could damage the CD player there are other mounts that do the same with less risk, other than the tell tale circular cup mark left on the window Former Mod/Member, with the most post & Chicken George Arch nemesis

I would be tempted to say it is unlikely to damage the unit. My understanding of the internals is that there is a sensor in the middle of the slot to tell if a CD is inserted. As the holder insert is the opposite shape.to a CD, the bits that goes in far enough are the sides so unlikely to trigger the mechanism.

I could be completely wrong of course :-/ Learned to drive in 2011.
Bought a Micra - great little car
"Upgraded" to Kia Sportage - hated it
Proper upgrade to FL2 TD4 GS 60 reg Facelift - tailgate paint issues, fuel gauge rewire, transfer box failure at 36k miles, clutch at 60k
Reconditioned diff at 70k ish
Sold at 84k and now on a DS. Terribly unreliable.
Sold DS for a 2021 Mitsubishi Outlander PHEV. Fingers crossed....
Sold Outlander for a 2014 SD4 SE Tech because I’m mental

I bet theres a stick on accessory cup mark you can buy from someone cashing in on e bay to make believe you can afford to have a sat nav, like there used to be stick on bullet holes years ago.
They say you can flog owt on e bay.

I think that I will get one to try out, they are cheap enough for an experiment, I will let you know how I get on.