From my second post above - "I usually wipe the top of the cap off with alcohol or brake cleaner to be sure the tape stays stuck."
In the pic is masking tape -- I've used it and duct tape with equal effectiveness. Once I discovered this method, truth is I grab whatever tape happens to be handy. Keep in mind the physics involved - hydro "head" is creating lower pressure in the top of the reservoir, and atmospheric pressure is trying to push the tape 'into' the hole. As long as things are clean, my tape has always stayed in place. Fact is - even removing the piece I applied for the picture took a bit of digging with my finger nail...
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