We went through several prototypes before we got it right. The earliest ones the caps would not fit everytime or sometimes only on side. We also did not anneal the very first ones made (annealing is a process that makes the glass stronger) Without being annealed the
tanks stood up to most of our drop test but not the more aggressive ones. Also getting a smooth cut was a challenge we overcame. In short we made a lot of tanks before we got it right. This the prototype tank I use, the cut is not very smooth and it was not annealed. I took the photo after it fell off the counter and hit the floor so hard that my cherry drip tip cracked in two but the tank came out fine!
It also has a small crack on the inside as our cutting methods for our first try were not the best!