Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4854094.
Simple 3D printable bag clip, useful for securing bags in the kitchen. The body is symmetrical, so just print two copies of the body and one of the spring clips. The first spring (1) seemed to work best for me, the second one (2) is a bit thicker so maybe stronger (but also a bit harder to fit).
I printed this in PLA (printing the body at 20% infill, and the spring at 100%). However, I guess PETG or ABS would be a more suitable choice for the spring.
For the spring, you can also try increasing the wall line count to 4 which should create a solid part, but will ensure that all the layer lines follow the curve of the spring.
Update 26/05/21: after 20 days, the springs printed in PLA seem to have plastically deformed (i.e. they seem to have set in a partially open position so, whilst still sufficient to hold the bag closed, they don't have the same gripping power they did when first printed). So, as I originally suspected, it looks like PETG or ABS would be a better choice for the spring.