1 1/8" forks in a 1" headttube, have a look at my IS41 adaptor and let me know what you think!

tester might have a good view

I don't know whether it's a good view, but I can see a Cane Creek EC34 headset and a set of digital calipers from here, and they tell me that the acceptable numbers are 0.7mm radial clearance and 2mm wall thickness of 6000-series aluminium, the first to avoid interference and the second to avoid shearing the plug off the flange.