A bit of cruelty

Other materials may include wire. Like this one; I made it out of jewellers wire, in the easiest way, because the wire is rather stiff to work with.

The easy way being: choose your size (10 cm for small, 20 for extra extra large, etc) and set up accordingly. Knit and purl with two needles, which makes the bottom curl up nicely like the ring of a condom does. Decrease after a while, depending on the size of your condom, quite rigorously but evenly over just a few rows, to end up with just a few stitches to knit the reservoir. Join sides together, and Bob's your uncle.