Heat shrink joining square and round tubing - GIF

[desbromilow]

aircraft modellers do a similar trick where they make up a small wooden form of a cockpit window, then shrink the PET bottle over the top to make the "cockpit glass", and then cut out the bottom to release the wooden form.
I've done similar to make ogive transitions for model rocketry.

The only constraint you have is that the PET won't easily shrink to less than the original "cast" diameter. PET bottles are made from blanks which are approximately the diameter of the neck where the lid screws on, so it's difficult for them to shrink less than that.