Sounds like you are using older stuff without a self-locking pin.
So first my warning - whatever you do, you'll probably end up with something sticking out of the rotating shaft that can catch your clothes & hurt or kill you. And whatever metal you use, it probably won't meet OEM specs for what would be acceptable. So when you hurt yourself, I'm the only one you can sue....
And I'm only telling you what I do, I'm _not_ suggesting you do this or it is in any way safe....
The farm supply stores sell pins for this purpose. You need to fit the hole in the pto. I'd get one of those.
For static loads like running an auger or elevator, I've often used any bolt or nail that fits. Probably not safe tho. For 3pt equipment that moves, I'd want to fill the hole.
--->Paul