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.)