Originally Posted by 531Aussie
I've done that. My derailleur was a bit old, so I just put in a cleat bolt and used a matching nut -- in other words, I gave up on the thread on the derailleur. You might also be able to use a flat-headed bottle cage bolt
A silver cleat bolt and nut looks good enough. Bicycle Recycle might have one: 9555 8692. Usually $75 for 2nd hand Dura-Ace, and maybe around $50 for Ultegra
Thanks Aussie, I spoke with the Father in Law who is an engineer and he thinks a stainless hexhead/star screw with a stainless nut will be fine and will look OK. He just happens to have a few 5mm of both in his work shop and is bringing them over tonight. I think I might also drive past your mate in South Road and just see what they've got, just for the hell of it.