The most important place to start is to give full credit to Thingiverse contributor "froland" who posted this treasure trove of lens filter adaptors:

https://www.thingiverse.com/thing:4544206

Without those threaded models, I could not have completed this Thing for all of you.

Almost everyone has an action cam (I have 4). Many people have actual GoPros, but many of us have bargain models which are really perfectly good for things like mounting a camera to my Robot Lawn Mower and taking a video of what happens when he bounces around the yard. (like this: https://www.youtube.com/watch?v=rZAPKCEVjKI)

What happens when you want to video your daughter's wedding using your Action Cams, and the chapel is basically a glass sunlight lens flare generator? If you were a millionaire or a social media influencer and you had high-end DSLR cameras that you could use to record videos, you would buy CPL lens filters for the cameras. CPL filters are polarized lenses that reduce glare and flare (like the better sunglasses many people use while fishing). You would screw the CPL filter on, and 90% of the lens flare and 98% of the glare would vanish after a little adjustment. (to learn more, check this intro to CPL filters: https://petapixel.com/cpl-filter/)

Your budget Action Cams cannot accept CPL filters. They can automatically adjust exposure but you can't add any kind of filter or extra lens.

Until now.

The CPL I bought for my action cams is this one from Amazon (and I don't make any money if you click thru to see it, or even if you buy it):
https://www.amazon.com/dp/B00XNMXRUW

It attaches to your bargain Action Cam using the rig I have uploaded here.

I am 100% sure there is a way to build an action cam holder that looks like the cheap one that comes with your action cam (the kind that "clicks" to hold the camera in place), but (like all GoPro accessories) that will break easily. Printing ones like that also break easily because you can't set the print up in a way where the layers add strength rather than create places where the layers want to snap.

Note that I have labeled the model parts "PLA" and "TPU." You have to be a risk-taker who likes to print TPU to make all these parts work together. The TPU parts are flexible and a little stretchy, and this will help a LOT. If you are a hard-core 3D printer and love the rigidity of ABS, I admit that you might like the performance of the PLA parts if they are printed in ABS.

I almost forgot: for the tripod base and the case back which both have GoPro-style connectors, you have to print the fingers so that the print layers run perpendicular to the compression direction of the joint. For the back it's obvious, but for the tripod maybe not. The fingers are off-set to the back face of the connector so that side is fully flat. Orient that side to the build plate when you print it.

The joint between the camera case and the tripod stand is modeled to take common 1-1/4" 10/32 machine screws and matching nuts (the nut set into the receiver). I didn't build plastic heads for the screws because I prefer to use a screwdriver. The foot of the tripod mount takes 1/4"-20 nut. I used a locking nut because I like the compression the flared bottom creates when you insert it upside down in the nut cavity.

All the model parts are uploaded. Photos are uploaded. I also replaced the filter mount model because I mistakenly uploaded an old version in the first pass.