Dunelm Mill have blackout curtains - I bought a pair today for 25 for the door to a cupboard I'm using to load film holders and processing tanks. the size is 46" x 54" but there are others. My wife usually puts velcro on them and I just stick them round the door whilst I'm in there.
http://www.dunelm-mill.com/shop/natu...lection-143158

cheers, Tony