Here's how I do it. Although I use this technique more often to make an original part from a master.
First, prepare a master. In this case it is the original canopy, and it needs to be pretty clean. That means gluing the crack, filling and sanding it reasonably smooth.
Support the master on a riser so that it's steady and clamped to something. Since it's sacrificial, that might mean filling the underside with glue or hardening putty and with a section of sprue or a stick in it.
Position where you can work at it from the top down, with clearance all around.
Take a piece of clear plastic and cut a large enough piece that you can grip it by the edges.
I use the flat tops from the clear tubs at the grocery store that salad comes in. Like a clear to-go tray with a lid. You can swipe one from the salad bar.
Take your piece and hold it over a candle flame briefly. It takes a few tries, but it can be figured out about as quickly as it takes to learn how to strectch sprue.
Switch to your master and pull the piece of plastic down over it with the soft spot over the master. Add a little extra beyond the bottom edges of the master.
It'll set in a few seconds.
I'll do a half dozen or so one right after the other in the same piece.
Then you can cut them out with scissors and trim to fit.
I'm surprised at how much fine detail gets transferred. Don't count on the plastic forgiving flaws in the master.
I call it plunge molding, Jeremy had another term. "Smash mold".
It's a good way to make dome lights, hub caps, bombs,stuff like that. You can practice on the end of a pencil.
I hope that's helpful.