Would have replied to this earlier but have just moved house and have had no internets till today...

I posted a R20 BikeCAD file on here a while ago that fulfills what you ask though you need to go to the free JAVA viewer/authoring tool over at Bikeforest to view it; But seeing as how the threads search seems to be down, I can't resist this sort of thing. Strangely - this twenty only has one strut between top tube and the seattube - kinda like the 'new' one...