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You know, it is an internet forum to discusss the info and tip. I am sure that many members want to hear this kind of info like myself instead of hearing "buying mine' and "this is a right one" at the discussion sub-forum.
Just my 2 bar.
I get what you're saying about this being a forum, but you are asking a manufacturer to give you tips on how to homebuild something he spent time developing. That wouldn't fly anywhere else, why should you expect it here?
Since once the valve is closed the pressure inside the solenoid valve and line will gradually build to the set pressure of the reg, and then maybe higher if the seat is leaky as many old regs are. So the a solenoid cutoff valve if fitted really ought to be rated for 02 and rated for a fairly high pressure. These can be hard to find and often expensive.
One way around this is to use a three way solenoid, set so that rather than cutting off the flow when the power goes off, it dumps the flow into the atmosphere. This wastes a bit of oxygen, but mean the O2 pressure in the valve never exceeds atmospheric pressure, so almost any solenoid valve can be used.
Someone always does. Finding stuff to buy is usually not difficult.I make a gizmo...]