Well Then - Where to start

Is it a one piece canopy made from clear plastic?
If so, the easiest way to make a new one would be to vacuum form it.
Assemble the old one (Using tape etc to hold it together) in the correct shape. apply some release wax (such as "Traffic wax" floor polish) Then make a "plug" by pouring some plaster of paris inside of it.
Once set the plug will need to have any flaws filled in and/ or sanded out of it as any flaws will show up in the final product.

A sheet of thin PVC, Acrylic, Polycarbonate or PETG (around 0.25 - 0.5mm thick) will be needed to make the new canopy. Your local plastic supplier can probably advise you on what is available and what temperature you should form it at.
This sheet will need to be at least 3 times the width of the original and at least 1.5 times the length.
Make a simple 2 piece frame to hold the sheet. The frame will consist of 2 rectangles that clamp the sheet between them.

Mount the plug on a solid base (Such as a 3/4" or 1" thick 25mm piece of wood) that will fit neatly inside the frame mentioned above (I use small "G Clamps to hold the 2 frame halves together). Before you mount the plug on the wood - cut a hole in the centre of the base board large enough to insert a pipe from your vacuum cleaner.

Mount the plug on the board with a gap between the plug and the board (I usually mount them on some thick washers - say 1/8" thick.).

Now for the easy bit

Insert (and seal) the vacuum cleaner hose in the "mold".
Mount the mold on a frame so that you can press the frame over the base plate
Switch on your oven to the forming temperature (try around 250 degrees F)
Get some gloves ready to handle the item (Oven gloves or welding gloves).

Once the oven is hot - place the frame in the oven until the plastic "droops" well. - switch the vacuum on before this time
Take the frame with the plastic in it out of the oven and press it over the mold - the plastic will form to the shape of the plug

Hold it there until it is cooled enough to hold its shape

I is that simple but buy a few pieces of plastic because the chances of a perfect job on the first attempt are slim - everybody has a slightly different technique.

This link is one of many that explain this

[link]http://www.marvin3m.com/vacuum/index.htm[/link]


Edited to correct the temperature Range (F not C)