Hi Boxhead
If you use Mini or Morris Minor/Oxford parts watch the dress rings. The original stainless dress rings on the older cars are slightly larger than the new rings. The new rings fit exactly to the body of your Cobra.
This is due to the stainless ring having a slight return back to the body.
Now the stainless button head question.
Check out
"khrace" - who Posts on 'clubcobra' he has Posted some pictures of his Daytona coupe build.
Under the 'Functional brakes scoop' photo in his gallery, you will see the button heads attaching the perspex scoop to the body.
Any fastener shop stocks these in all sizes.
Cheers
Bernie