While I have no desire to take the Osmo Action below 11M, I always felt better when my GoPro was in a housing.

I know DJI hasn't even announced one, Does anyone know of a 3rd party unit? Do the Hero cases fit (I have 3&4 stuff hanging around).

So if you know, please let me know

Hello and good day TheCameraGuy. Thank you for reaching out and for the inquiry. I hope that you will get the best information with regards to this inquiry from our valued DJI members and DJI customers. In addition just a friendly reminder that we do not recommend you to use a 3rd party accessories or devices on your DJI Osmo Action. Actually, there is a  possibility that this accessory will work, however, since it's not yet tested by DJI we cannot guaranteed the reliability of the said accessory when in use. In addition using 3rd party accessories or devices might cause harm on the DJI Osmo Action and can void the warranty as well. Thank you for your understanding and support.

Have a look at this one Osmo Action Cage it’s pre order but a very good company and usually deliver ahead of pre order date. I have several of the cages and wooden grips for my other gear.

Just re read your post, sorry I didn’t realise you meant under water housing.

I like the small rig cage. The mistake is the $80 Arri handle. It costs more than the rest of the kit