I use the clear and black and film canisters in all my thumbnail tanks for breeding purposes, and the frogs always leave their eggs in them. My problem was that the frogs always deposited the tads in the water area, which normally has a dark colored bottom, and I have a difficult time finding them, unless they move. Recently my wife suggested that I try putting a shot glass in the tank. I tried it, and within a few days I saw the male inside the glass. Low and behold!
Shot glasses really work. (Note: Probably better to use a clear one, instead of one with a Tequila label, LOL)
I prefer glass as opposed to plastic for just about everything. It just feels better / heavier and is much less apt to tip over and loose the tad than a small plastic food cup of some sort.
Do you feel that the shape and size is of any importance?
I am not sure if the size played much of a role, but the ones I used are pretty unique. I collect shot glasses from everywhere I visit. I like the taller ones.