I have several that I made from paper clips, the larger size.
First I connected both to a common stem (like a "Y") then I decided that longitudinal (like a "U") was better. Jury is still out on that.
On beasts like Model Ms they work great, but on some tighter keyboards I have trouble getting between keys, and especially fishing the "Escape" out of its lair.
And since paper clips are such low-grade steel, they bend like crazy if I am being the least bit rough.
I really like that guitar string idea, I need to remember who I know that plays, and who will give me an old string.
Now I am trying to imagine other sources for wire/string/cable/strands that are "just right" .......
Occasionally I employ a butter knife or screwdriver, but only when the situation demands it.
I have removed thousands of keys, with great results, it seems that 99% of my fumbles occur on spacebars. I have wrecked a bunch of those, and it really irritates me, because they have to be "right" because I will either be keeping the keyboard for myself, or selling it on ebay, where I strive to keep a 100% positive rep.
I doubt that any of you have found much different.
Since my current infatuation is the IBM Model F, I have been wondering about those plastic "cleats" or "tabs" that hold down the wire stabilizers for the large keys.
On most keyboards they are molded into the "backplate" or whatever the thing that holds the key stems is called, but on the F they are inserted through holes in the metal plate and melted down like the rivets on Model Ms, a proven bad system.
Is replacement with donor parts, held in place with epoxy or some other adhesive, a reliable long-term repair?