You know, I see some negative comments on here. I disagree. While the design may not be perfect, and there may be better ones, but I appreciate the designer making this and uploading it for me to try. Mine came out great and the clips that hold the rubber bands press fit just fine. If anyone said they didn't fit I'd say they probably need to tune their 3d printer better.
Ultimately, you weren't forced to download this and I don't think any promises were made that this would be the next sniper rifle. Be thankful.
Also, in regards to it holding a penny, It has to be able to release the penny so I'm sure it can't just clip in and hold it tight.
Anyway, thank you Zheng3 for sharing. Posting a Make as soon as I'm done here.