PET Bottle coupler / Hourglass connector

I originally made this for a LARP event that needed a bunch of very large hourglasses, and we made those out of clear 2L PET bottles and very fine, free-flowing, sand. 

We used the smallest version at 3.2mm to get a reasonably long runtime, but I've included multiple sizes up to 8mm so you can use the one that works best in your application.

2024-01-04: by request from @ericderace, added a stronger version with 2mm extra wall thickness.