I was thinking, the easiest way to make a copy of a canopy or other vac formed parts would be to make the plug from Plaster of Paris, but, what do I use as a mold release agent to prevent the plaster from damaging the part? I have a few parts I want copied so I can duplicate them in case my thumbs do something dumb, and these parts are no longer available.

I'm no expert but I think 3-D printing might be a good way to make a plug if you know someone with the equipment and skills.
Steve

No, I don't, and already have enough toys with the Laser and Sherline tools. I don't have the time or patience to learn 3d modeling either.

There is a product called polyvinyl alcohol. commonly referred to as PVA. It can be sprayed, or brushed on. There should be videos on the net on how to use it.
Fiberglast Products has a number of "how to" videos, so that might be a good place to start.

I have done this very thing in the past with unsatisfactory results. A mold needs to have a glossy, polished finish. Especially if you expect to have a nice clear, not cloudy looking, canopy pulled from the mold.

I made a few molds with drywall mud. The powder kind that does not shrink much and has to be mixed, or plaster of paris. I painted it a few coats with a thinned enamel which absorbed it well and left a fairly hard smooth finish. I have heard that Pam cooking spray is a good release agent, but will admit that I have not made any parts yet after a few years of sitting around.

This is the route I am actually planning. There is a 30 minute drywall mix used to put a finishing surface on, without sanding. Hard as a rock. The trick to getting a good solid clean cast is to make sure there are no air bubbles, so I have a vibrating table in the works to shake the piece as the plaster hardens. Then a few coats of sanding primer, wet sand, and a finishing coating of clear coat, and it will work, I just wasn't sure about the release agent, so will give PAM a try. I'll have a test piece done first with a windshield I have laying around.

I didn't use a primer. I just wanted the enamel to make a harder surface and soak in as deep as possible. Sanding would change the shape of surface lines, if there are any of course. I used Durabond 90 and Sheetrock 30. Only because I had them around. I think the Durabond is a bit harder. Harder to sand on the walls at least. The back of a vibrating sander should shake out any bubbles. I just tapped it a bit with my fingers.

The canopy is painted inside, so there are some runs and brush marks I will need to fill in.

The original needs to be well supported so you'll probably need to embed the canopy in something like a box of packed sand. The mold material will be fairly heavy and tends to distort the plastic canopy resulting in an unsatisfactory plug. PAM does work well as a release agent. You may need compressed air to help separate the plug from the original without damage.