When I had a 20mm front end I used a Hayes 20mm, 203mm adaptor. It's about twice as thick as a normal adaptor. Post Mount only though.
Are you going to run a chain device? Just asking as I may have something for you Is it a BB mount or do you have an iscg mount?
Alas the steerer isn't sorted yet :?
One the downfalls of bank holidays is i'm not in work to do my tinkering
I do however have a section of steerer on the way so should be sorted soon
Now, it does need a bottom roller but any chainguide roller will be ok (I expect someone on here will have one if you ask). And the seat-tube clamp is missing (same goes for this or buy a new one for £3.99 http://www.chainreactioncycles.com/Mode ... odelID=745 ).