Vaccuum forming
The best thing I have found for copying a canopy (making the plug) is Durham's rock hard water putty, you can get it at like Home depot. Buy the big can, it's a powder you mix with water, mix a big enough batch to fill the canopy all at one time. Make a frame to support the canopy. Once you fill it keep tapping it against the table to get the air bubbles out. It will take 24 hours to cure and it will pop right out. Sand or file off the flashing around the bottom of the plug and give the plug a good sanding with 400 + grit paper otherwise you will have problems pulling a vaccum under the plastic. This is the best stuff I have found for the price, the best thing is Hi temp metal filled epoxy big $$$.
Lance