...I made a small bracket to hold the coil. I was making something more sophisticated, but decided to just do a simple mount that would use hose clamps, as that seemed more in tune with the character of the bike...
Yeah, period correct, unfortunately, but cheesy.
Similar clamping challenge in this racer restoration:
http://www.xs650.com/threads/xs650-road-race-restoration.36433/#post-367713
Were you considering some sort of band clamp(s) that would work with the aluminum block, like these?