Looking for advice - from TG4 to...

Is it this one: https://www.aliexpress.us/item/3256804532653545.html? That could be a good option if it works well - the single flip adapters from Backscatter are $165, which is crazy.

Yes,@webcubus, that's the one. Let's see how it works. Honestly, some prices are just crazy for a bit of aluminum and inox screws.

Have you tried it out yet? I got my rig and a diopter and just need the flip to complete the puzzle.

Yes. But I am not superhappy with it: thread seems to be slightly oversized and, originally, I was not able to fit it into my old polycarbonate housing. 

I finally decided to be a little bolder and used several times my beloved inon diopter on the thread for machining it. Inon diopter has burping slots that where perfect for this as the borders are sharp. 

Now the flip holder goes tight, but it is usable. Forcing it into the housing when new could have caused a disaster.

I found another solution. I printed this and grabbed some hardware at the local hardware store: https://www.thingiverse.com/thing:4129223. They didn't have M2.5, so I went with M3 and drilled the holes a bit bigger. Everything fits nicely and doesn't seem like it's going anywhere, but I used some monofilament fishing line to attach the diopter to my tray just in case. 1 dive in the books and it works like a charm. The 60mm + flip diopter is an excellent combination!

Nicely done!  Thanks for sharing the link