Buy a fruit fly trap for €13? I said no to that a long time ago. I always make them with a piece of paper and some sticky tape, but that is just super annoying to make and can be used just once.
Then I remembered; I have a 3D printer! So I looked up a model of a soda cap and made it suitable as a fly trap. It hopefully saves you a lot of time and headaches too! It is a lot more nature friendly this way, since it is reusable!
I have printed them with PLA, 7% infill. No supports needed.