In the spitfire movie (this one: https://www.youtube.com/watch?v=5A_HJXmwLQ4) the one with the blue linings on the wing has a nice bubble canopy. Is it for sale as a part for this aircraft?
You can use the poster board canopy and cut out the inside of the design leaving about a 3/16 outline so you have a frame then laminate it using pocket laminate found by the school supplies at wal mart then cut it out leaving about 1/8 in of laminate all the way around the frame and then just hot glue it on.
I left it as an open canopy because of my removable hatch but have done it on another plane and made it in 2 parts because of the mulitiple bends then just glued the 2 together.
I also found if you go and buy BIG battery Heatshrink. You can take planters foam and mold your shape then form the heatshrink with it for a nice custom canopy as well.