Because I found that an M18 nut will fit snugly onto the right, and an M20 onto the left, of the quick release back axle. I will have to grind a slot a few mm deep into the M20 nut to allow for the quick release lever – and hopefully won’t damage the blue locking inser (because it seems to provide some cushioning and grip*).
But it’s not simply a matter of drilling holes in the existing stand for some threaded-bar clamps. Because the current stand is far too narrow. So I think I’ll replace it with a built-for-purpose stand and weld that to the generator’s frame.
Then I can freecycle that uncomfortable mountain bike out of my life! 🙂
* The blue locking insert may not provide enough cushioning against vibrational / noise while pedalling. If so, I’ll try inserting some rubber washers. Or might step up the nut sizes to allow for more rubber to be inserted (e.g. M20 & M22).