I've ended up using 5/16" extruded u-nuts and matching 5/6" x 1" body bolts with the large loose washers included. Metric sizes were simply unavailable anywhere I went. The ones I'm using are made by Hillman and are available at Pep Boys in their small parts/hardware/fasteners drawers (these may not be on the retail floor, so, you might need to ask an employee about them).
The drawers are a mess, so, definitely look around in the other slots and in the drawers above and below. I was able to find 9 u-nuts in one store doing this, and 0 in a second store.
NAPA seems to sell a pretty much identical model of u-nut made by Balkamp, and I plan to check them if I strike out at the last local Pep Boys in my area. NAPA's are actually viewable on their website. The bolts I'm using look like this (https://www.napaonline.com/en/p/BK_6652324
), and the u-nuts look like this (https://www.napaonline.com/en/p/BK_6651303
) - but these are metric 6 examples - it's a real pain to try to find specific sizes of these on Napa's website....
Home Depot sells a u-nut / speed nut I looked at too, but I did not like the design and I don't think it would work quite as well as these one's from Pep Boys have.
Hope this helps!