This particular model is very tight so make sure you have fastened it to the rail before installing the USB port. Do not overtighten the screws or the holder will not flex enough to allow the port to seat. I used 4-32 screws but M3 will work fine.
The author remixed this model.
When I ordered the USB extension from the Amazon link provided, I was sent a different model, so I designed the new file from scratch.