Morris Minor Owners Club

Roll up a piece of sponge rubber and stuff it between the bolt head and over rider body.
This holds the coach bolt in place while you fit a nyloc nut.
Took me an age of upside down faffing with the bolt falling out every time I started to fit the nut before I found the obvious solution!

I've just had the bumpers apart on one of my Minors to respray the valances and so have just had to put the overriders back on. I didn't have any problem pushing the bolt through and simply holding it in position with my finger (from the bottom of the overrider)while I attached the nut.

I should perhaps have mentioned that I have limited sensitivity in my digits due to white finger syndrome and the sponge rubber holding the bolt securely made the fitting that much easier.
Your method is of course correct for most people.
Apologies for any confusion.