12" x 2" x 0.5" steel plate
Cost me $3 at the local scrap yard.
You don't see both at the moment ...
but I drilled two 5/8" dia. holes ...
after I laid out the two mounting tabs across the steel.
BEFORE I cut the tabs off ... because my little fingers wouldn't be able to hold that little tab when it wanted to start spinning ... and I am allergic to messy blood and missing finger parts.
(Been there ... Done that!)
Why 5/8" (15.875mm) and not 20mm?
I don't have a drill bit that big.
The large rattail file and I will have to enlarge the hole slightly to accommodate the 20mm axle bolt. (sigh) Jack tried to teach me better than to do all that manual labor ... but sometimes you just don't have the appropriate tool ... :-)