Thruxton caps fit on a speedmaster!!!!
Adjustable fork caps from a truxton are a direct fit to speedmaster and america 41mm fork (03 - 09. Only mod is you have to cut down your spring spacer to fit. The adjustable one is slightly taller. Also you will need to add the metal disc that goes between the cap and spacer tube. Gives the adjuster on the fork cap something to push on.
They sell the adjustable fork caps at dimecity cycle wedsite. Not sure if they come with the metal disc but you might be able to use a motor valve shim (looks vary similar to our shim over bucket valve shims)
http://www.dimecitycycles.com/media/cata...-t2049009-1.jpg