If I look for somethong with a magnet, I hope to have another piece like I am looking for and see if it is magnetic, then go one step farther by let's say throwing it in the grass (Where I can see it) and see how much luck I have recovering it. No sense pending time on a plan that isn't going to work.
Yeah, I'm familiar with that technique, however.....
with these particular clips just taking them OUT can lead to them springing apart, similarly putting a test part back in, so I am loath to sacrifice another one just to see how/where things fly, land and hide.
ANYWAY.
Today I put one of the plastic rivets in, the 1/4 inch hole one was the best fit, the 7 mm one was too fat.
Another thing I liked about the 1/4 inch hole one is the large head on it, about the size of a US quarter.
I didn't run the tractor to see if I could get it to vibrate out, I was there for other reasons including putting the wheel back on the Harley rake (new inner tube).
It will be no great hardship to have to carry a #2 Phillips head screwdriver on the tractor<grin>
When I have nothing better to do some winter's night I may take one of the clips apart, lay the tab on my scanner to get a profile of it and then print a few in ABS plastic (bicycle helmet plastic), which is good tough stuff. About 1mm thick should work.
I know, I know, I'll NEVER find one of those with a magnet.