While there's a lot of fantastic advice on here about how to do this right, in a pinch and when the forces are in the right directions, you can get away with threading directly into a piece of plastic. As an example, we used 100% fill ABS printed pieces as clevises (clevii?) on the 'Snow Problem catapult. Here's an
imgur album with pictures of the implementation. Personally, I wouldn't recommend it, and I'm somewhat surprised they've held up for the couple hundred shots we've taken with Perses, but in this particular use case, it worked out to thread directly into the plastic.