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I faced the same issue a few years ago when specifying replacements for Talbot Talflo taps which after several years of use had worn sloppy, are not cheap and are heavy for air transport. People operating these taps have a lot of time to think how to fix them open while they wait for their containers to fill and I have seen them tied up open with bits of vegetation string.
The option I found was from the suppliers of much of the emergency water treatment kit for OXFAM: Evenproducts Ltd (now acquired by KAR UK Ltd ??). Their taps (EVenflow valves) re a cheaper option and are much lighter in weight. They are mounted on a horizontal spigot and have a horizontal push to open action and a spring assisted closure. I sent a bunch of these out to DRC some years ago but have not heard how they fared - so I need to check on this.This design is also susceptible to being tied in the open position. See http://www.evenproducts.co.uk/pdfs/Evenflow%20Valve%20Apr11.pdfhttp://www.evenproducts.co.uk/pdfs/Evenflow%20Valve%20Apr11.pdf
I too spent some time thinking how to make a simple, cheap tap but I never refined it. One of the simple options was to pinch a flexible tube but the life would inevitably be short and this is where one of the the trade-offs can be found: cheap but low life or expensive and longer life.....
I appreciate this does not meet your design criteria but is an option to the Talbot Talflo tap.
Have you looked at the Tippy Tap?
All the best. //Steve