I don't use sticky foil as I find them cumbersome to work with, plus I like to do things as inexpensively as I can. I use mate black thick paper (in the USA is referred to as 'construction paper'), such as used for kids to do crafts at schools or home. Sold by the sheet, for very little at hobby or paper goods shops. Cut to size (as close as possible so as to fit snuggly between the top and bottom edge of the plastic trim in aquariums), then affixed in place using black electric tape. Have six aquariums done this way and some of them have been in place for ~5 years without any issues. Cheers!