I chopped up an old tandem to make my whip.
I started off with a GT Quatrefoil. Nice cro-mo frame but it was huuuuuge.. and the daft 700d wheels didn't help. Plus I wanted full suspension for my child-back e-tandem, so... Stripped...
I chopped the rear stays off, and then chopped a foot out of the centre square. This took some care to get right, due to the geometry.
Then I made a mount for GT STS rear suspension, welded it all up, painted it and fitted it up to my chosen specification.
The bike is, for want of a better word, awesome. Every day.