Most of the pressurized rocket body vessel is covered with a second layer of PETE plastic,which almost doubles it's wall thickness. Let's face it, the old method of attaching the fins to the rocket is a task, and it's a huge disappointment when there is fin damage to a rocket, disabling that particular rocket. Or look at it another way, if you crash and damage your rocket body, you are left with a damaged rocket with a good fin assembly. Either way you loose. The circumference splice joints are better and hold higher pressures.
All parts fit perfectly.
Using heat to curl cut edges of bottles greatly increase the strength of components. Both have a specific purpose. The FULL CURL is used to reinforce the cut edges of bottles. It is essential for providing a tight seal for the water tight compartment.