You didn't specify which model you have. For mudguards (fenders) the main question is do you have suspension or not?
If you don't have suspension, then any mudguard made for 20" wheels should fit. SKS, Zefal and Planet Bike make high quality ones.
If you have suspension then you'll need to adapt one of the above to fit, because the full suspension Downtubes don't have mounting for mudguards (or at least didn't last time I read about it).
Velochocolate (
http://www.velochocolate.co.uk/) is run by one of the forum members here, and were looking at providing a kit to fit Downtubes. I don't know if that is currently available - the web page isn't working.