The problem with making your own reflectors is that a lot of the aluminum available in stores is not reflective enough to give you maximal light reflection.
You can go to a local home improvement store and purchase a sheet of aluminum for around $20. Get the most reflective one you can find. Bend it to fit in your canopy. Look at the ones in your LFS and try and copy their design shape.
When it is shaped, get a polishing compound (also at local home improvement stores or hardware stores) and polish the reflective surface of the aluminum. You may want to ask someone at the store to see what compound will work best.
Hope this helps.