So far I've been unsuccessful, too. I've tried using transparency sheets as well, but I think that black ink printed on transparency lacks the opacity to effectively shield the treated fabric from light. It seems to be only functioning as a sort of neutral density filter that blocks the light a few stops, but can't hold it back completely.
(I have an inkjet printer, and inkjet transparency film is difficult to find here and is quite pricey, so I was using inkjet window cling sheets, which is transparency film coated with a low-tack adhesive on one side.)
You can maybe stack a few transparency sheets together to create a darker layer. I have used this alternative process with a laser printer for my stencil work as well as for photograms.
Too bad this product isn't that sensitive, I would have liked playing with some. Maybe a future version will be better. For now I'll stick to hand cut stencils and spray paints.
I haven't tried it yet, but how about Lithographic Film? That seems to be quite dense.