A few clarifications- I have a couple thousand hours in Navy jets and quite a few private plane hours also, so I know what I'm talking about. The canopies are durable as were the windscreens in most WW II aircraft. They were all cut and drilled many times (stress points) during manufacture- if I were to count the number of holes around the edges used to mount just on half of a canopy (there are four on the EA-6B) - they would likely number close to 50.
As far as differences in application- not much- high velocity air often containing small particles, including wood chips (shop) and sand (Iraq, Afghanistan) impacting the plastic.
As I understand his rationale for using plastic, Ed built a Pentz cyclone for himself. He liked it so much that when he decided to build one for his son he wanted to see what was happening inside. He had experience in fabricating metal and plastics but thought that with his skills, woodworking tools, and some basic molds and heaters, he could make it from plastic easier than from metal.
But just not any plastic. He uses PETG (a thermoplastic polyester) which if you check the properties is well suited (likely better than Lexan, a polycarbonate, or Plexiglass, an acrylic) for THIS application. PETG is tough, machines well, can be solvent bonded, and has a low forming temperature. It was no longer a "novelty" but something Ed could easily and quickly "manufacture" with woodworking machines, a few jigs and heater units, without the need for welding or other special metal fabricating tools, and did not need priming and painting when done. Sounds like a wise decision!
Now as to the OP's problem. I think we might have a terminology problem. If he has a "crack"- an actual failure of the PETG then by all means Ed should replace it.
But . . . what I believe he has is an open seam- and overlap that has separated because it was possibly not bonded completely. The ClearVue mini top cylinder and lower cone are made by heat forming sheet PETG. They have overlapped seams which are solvent welded together. The cone is also solvent welded to the cylinder. A "crack", really a small gap, in this context, could be easily and adequately fixed with either caulk or solvent welded together. Remember, in most applications it is under suction so any caulk will be drawn into and seal any "crack". The solvent, Methylene Chloride and others (IPS is one brand), typically used to bond acrylics and other plastics is available at plastics stores (ACME, Reed, etc.), craft stores, and Ebay.
And yes, I have ClearVue mini, so I know how it is made: