A foldable plane made in a single flat print that surprisingly does fly. I tried distributing the weight around and found that you get very different results depending on what you do or how big you make it. The version I published is by no means the best it can be so feel free to experiment with it, the files are available. 

I printed it with PLA because I have no PETG at the moment, but I think it might be a better option because the PLA ones just break after a few throws.

Printed on a Prusa Mini, it is basically just a few layers.