I am using John (
Narked@90) manual button at the moment. It is nice in the sense that you do not need to own any additional surplus dive gear to make it work.
It takes a few minutes to cut the hose to proper length, add the end adapter and screw straight into the parts. Everything is supplied. If you want John to supply with the hose already installed for plug-and-play, it is also possible. And would only take less than 1 minute to install.
If you are looking for alternative down the road as I am right now, I will eventually swap the manual button with one of my surplus KISS valve and connect it to a 2nd O2 tank (i.e. 1L). Benefits and drawbacks are self-explanatory...