Here is the PDF for the product I used:

It says "for room darkening applications" and that can mean many things. As it came, the top cassette was not light tight at all. It required extensive modifications to be light tight. In the end it worked out OK seeing as it was the only product that was available to me in the USA. After spending quite a few hours sealing all the little cracks (not much different from sealing up light leaks a monster enlarger head) it is now totally light tight.