1) first get some tracing paper and design the shape you want.
2) then use large index cards to cut out a template from your tracing paper.
3) then go to home depot or lowes and get "lexan plexiglass." should cost about $15-$18 depending on big you get it
4) tape your template to the plexiglass
5) use a dremel and cut it out
6) sand down the edges to smooth it out
7) wrap it in vinyl or paint match it
doing it this way you skip the long tedious curing part if you used fiberglass, resin and body filler