I'd tried a handful of SteamDeck mounting kits, and the DeckMate (even the version I printed at home) stands out above the rest.

https://deckmate.me/

I made my own mount that would connect to the ring-mount, and featured VESA-compatible prongs that I could use for additional accessories, if desired. 

I printed the support "beams" of the deckmount with a rough texture (called fuzzy skin) in PRUSA Slicer, so it would have a little more friction when clamped onto the rings, and that (plus some rubber washers) seems to hold quite nicely.