The plastic adapters for equipping a clipless pedal with a platform already exist. These are usually difficult to get in and out of the spring mechanism and are not the most stable. This is where pocket pedals step into the breach, with a different approach to how a clipless pedal mutates into a platform pedal:

This robust plastic cover is initially flat and when it is pressed on, it can be slid over the pedal body. Pins on the base are intended to provide grip, and recesses for the protruding parts on the clipless pedal ensure that the cover sits securely on the pedal.

If you want to fit the Pockets pedal to your clipless pedal, you can order online:

pocketpedals.com