[Andrew]

The van door shaft diameter is 11 mm (.433").

This is almost 7/16" (.4375").

We have machined an 1/8" key way in our motor shaft.

You can then take a 5/8" coupler, drill a 7/16" hole through (using coolant and pre-drilling with the one size smaller drill first to keep the hole diameter as close as possible to .434"), and then broach a 1/8" key way in the coupler. 7/16" is a standard bushing size.

Or, you could go metric all the way. The only difficulty is that 11 mm is not a standard bushing size and you would have to make a custom bushing for your broach.