You already have one of these.
If not, ask your friends, they have one.
If not, go to your recycling center, they have one.
If not, go buy one.
Note: there are at least two varieties out there: one with tabs that slide into slots (see image), and one that's threaded and actually screws on. Find the kind with the tabs. You could use the threaded kind and it would work just as well, but you'd have to make your own notches, and who has that kind of time?