When I switched to using an acrylic build platform, my retainer clips (designed for foamcore) stopped working. I worked with paperclips for a while, and then discovered OpenSCAD.

This is a very simple clip model that can be parameterized with your required dimensions (see the first few lines in the scad file).

In the pictures, you can see two different variations. Both have the same internal dimensions (the Gap parameter), but I varied the Reach and Thickness parameters. They both fit perfectly, but the smaller one prints faster and has less risk of hitting the z-stage. It's what the SCAD file defaults to.