I got a couple of recticel 25mm boards from Jewsons:
Covered them in lining paper using wallpaper paste, painted one side with blackboard paint, the other side with white matt emulsion, and followed this video:
The hardest part was finding the 3" gaffa tape in the UK, seems its only really available in the US, I managed to get some from a German company in the end.
EDIT: Took this while under construction:
forum: Flash and Studio Lighting