...a door hinge pin.  Fits into the pivot pin holes almost perfectly.

Now why didn't I think of that.

Thanks, Midnight!

So do pencils and allen wrenches

I don't know who originally posted this, but for $1 you go and buy a 1/4" clevis pin.  Buff it down lightly to remove burs, and maybe ream the holes so the detent will drop through (sometimes there are metal burrs in the holes) and you have the best pivot pin installation tool money can buy.  It turns the "hardest part of a lower build" into a ten second job without risk of sending the detent to parts unknown.  

I wish I saw this post about a week ago.  I almost put my freaking eye out.

I used the butt end of a 1/4 inch drill bit when I built my M4.  Worked pretty good.