A parametric clip to seal those bags closed that don't come with a seal on their own. Keeps food items mostly airtight, and smallthings in their bag.

Experiment with the parameters length, width, height, wall thickness, clip width and spacing until your bag clip looks viable, or choose one of the two known good configurations (for either 30mm or 50mm clips). Once you have a working configuration, resizing the clip with the length parameter is pretty safe to work. The spacing parameter determines how thick you expect your bag material to be.

Print, remove artefact bridges with a scalpel. Find bag, clip together.

The clip comes in two parts, which interlock by means of the two side clips and a profile along the centerline. When not in use, just clip the two parts together. Or fetch a piece of string and print http://www.thingiverse.com/thing:5627 instead.

You are very welcome to post your working configurations (both openscads and ToM parameters) in the comments. I can upload more stl files if asked to.