To expand on Kens idea,

You can carve a balsa canopy to the shape you want.
Mount the balsa canopy on the end of a 2x4.
Then, take the 2 liter bottle and cut the ends off.
Next clamp the plastic in between a couple of 1in.x2in. sticks on each side.
Take your heat gun and heat the plastic until it is soft.
Then just pull the plastic over the balsa canopy.
You may need to continue heating it as it is pulled. (it helps if you have an assistant to heat, while you pull)
Trim and install your new canopy!

Sounds easy, but it will probably take several attempts to get it just right.
I have been using this technique for over a decade. It does work well.
It just takes some practice.

Doug