Personally - I'd go with a existing, fairly common, universally accepted system.
For example: GoPro mount.
You can get GoPro mounts, adapters and accessories all over. There's tons of them ready to print. You just need to work out a couple semi-short arms to run a bolt through and make a sort of... double hinge to be able to fold the ocular all the way back or forward.
Plus you'd be all set to swap out the UA you're using. Even put a camera on it... Different UA plate... whatever.
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