McMaster-Carr catalog (http://www.mcmaster.com/
Pages 1077-1080 have all sorts of flex couplings of various kinds. I
don't know that any of them is an exact match for the application,
but there's lots of good ideas for ways to build them.
The Schmidt coupling on 1078 is one way, but the ones there are too light duty.
The double loop coupling at the bottom of page 1079 is one scheme.
The "tire coupling" on page 1080 is real heavy duty. One could
easily improvise something like this or the double loop with the 4
bolt pipe flanges and some pieces of tire.
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