It depends on the plant. Neoregelias and some other broms can be mounted by simply poking the stolen directly into the gs. Other epiphytes can be mounted by pinning them, with of without sphagnum around the base, depending on the moisture needs of the plant. Climbers are usually just planted in the substrate and allowed to make their own way up the bg. Also, pots can be sunk into the gs (make sure there is adequate drainge). As far as film canisters, no, they are not necessary, but are helpful. I personally use pill bottles from the pharmacy (must be very well washed). I would argue that the quality of light penetrating them is more similar to that in a leaf axil than film canisters.