I have seen cpl's listed for other dashcams and action cams but they typically do not give the size of the cpl filter or the i.d. of the mount that allows measurements to compare what else it could fit..
The problem I see with your version would be limited to dashcam use. Many mobius (any version) are found located behind the rear view mirror or another discrete location that is not easily seen by the driver. Many times when trying to install a cpl the installation would be placed off center a few degrees without an alignment tool being added. That is why the flat piece was added to the top of the mockup. If I understand correctly a cpl being off just 3 degrees loses effectiveness.
EDIT: You could glue on a (mostly) flat bar to your current cpl setip and have what I am talking about.
As for purchasing a CPL for an aftermarket DIY purpose the best thing to do is look at the standard specs such as filter diameter and lens thread dimensions.
So, why would my Maxi CPL be limited to dash cam use? I don't see your point. The only challenge to using it for another purpose other than dash cam use would be adjusting it properly for the type of reflections you want to eliminate or effect you want to achieve since you don't have a screen. Personally, I have no interest on using a CPL on an action camera like the Mobius if it is not being used as a dash cam.
As for dash cam use once you adjust and mark the proper setting for the CPL you can
accurately return to that setting very easily. If you remove the CPL from the camera and then reinstall it all you would need to do is make sure the adjustment marks are aligned. This can be accomplished with the camera in your hand or installed in its mount. I don't find it to be much of an issue. I've done that periodically with the rotatable CPL I have installed on my varifocal Mobius.
@Harsh marked his lens cap CPL and lens with small aligned pieces of tape and his approach appears to make it very easy to accurately set his filter. If the camera is behind the rear view mirror, place the markings on the bottom of the lens so you can see it from below for the moment or so it takes to twist the lens cap into alignment.
As for what you believe to
"understand correctly" about how CPLs work, especially with dash cams I would suggest obtaining some actual hands on experience which it sounds like you don't really have much of here. You are starting to sound quite a lot like c4rc4m who hands out advice and "suggestions" for others to pursue that he only has vicarious experiences with.
As for adding a "flat piece" to your, umm, "mock-up" it sounds like you want to create some sort of contraption. K.I.S.S. To be honest I don't really know quite what to make of what you posted. What exactly is sloppily wrapping a camera in a piece a paper supposed to be conveying here? I don't get the idea of presenting a photo like that and then suggesting it as a "design" that could possibly be created,
"if anyone wanted to make it happen". If you feel you have a solid concept for a DIY project you should make it actually happen yourself or do an adequate proof of concept and show us how it would work. That's how it's done around here.
As for gluing some sort of flat bar to the CPL I've created here, I have no interest in doing such a thing. I don't see it as a good idea or in any way necessary. It also would complicate a simply workable design. But, please, be my guest and make your own working verison of this CPL and show us how your idea is supposed to function.