This is my second draft of a cardholder/wallet that can be attached to a belt or a shoulderstrap.
I've done a test-print of the first draft in White, Strong & Flexible (WSF) and it worked out very well. A minor issue with a gap between the lid and the opening it goes into needed to be fixed and the model has been updated with that improved version of the lid.
I'm planning to make a modular version so that the springs and the lid can be made into separate objects.. possibly even separate front-plates objects. That way you could print the main "box" in one material, and the springy parts in another material (any of the "Strong & Flexible" materials).