My late father made me a pair of tiny hardwood wedges for the windows on our Thirty, push them in up by the hinge, between the window frame and the rubber seal. They keep the windows open and no problem with them closing. Just remember to take them out before trying to close the windows!
Been using these since 1990 across all my minis, they live in the door pocket and are as much part of me as my cars are. Lovely sunny weather brings back happy memories of my dad whenever I use them.
A solution doesn’t need to be expensive or complicated.
Not sure I can match your late father's skills but taking your post as inspiration, here's a couple I made from a clothes' peg.
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