The UK suppier is http://www.speedgraphic.co.uk/diffusers_filters_etc/better_beamer/12837_p.html
I picked one up from eBay for £18 a few weeks ago.Basically you get 2 side panels, a piece of velcro and a lens, due to a neck injury I have'nt had a chance to get out and use it,so I can't comment on how good I think it is.
Have a look at this site?
http://www.glennbartley.com/naturephotography/articles/Gallery - BCCHURCHILL2011.html