After becoming fairly confident this method would work on a completely broken piece, the two pieces were cleaned with soapy water and rinsed. Please note that clean plastic when doing this sort of repair is quite important. If there is dust and dirt on the plastic, it will become part of the bond and weaken the plastic.
Plenty of cement was applied to both pieces to be bound together. The broken edges were not sanded smooth. The goal was to help align the pieces properly by fitting the broken parts together the way they broke. There was some resultant deviation in the final bond so perhaps sanding the edges smooth would have been a good idea.
An oversize fiberglass patch was cut for this. I wanted plenty of support for this.
After applying not one, but two patches of fiberglass and saturating them with cement the entire thing was held together by your's truly for about 15 minutes. Any sort of more stable wire or clamp based support just seemed overly complex. After 15 minutes of curing the piece could be set down on my work bench so it could complete its curing without holding it together.
After curing was complete, the flash plastic on the finished side was trimmed with a razor, the whole thing sanded with #180 grit sandpaper, washed, rinsed, dried and sprayed with three coats of Rustoleum Flat Black Acrylic Enamel. It is the same used on my front and rear fender.
In the final outcome of this break repair, there could be improvement. I could have sanded the joint a lot more for a completely smooth surface. My goal here is not perfection but experience.
Please note... Just before taking this picture I ate a breakfast of pancakes with powered sugar. I get my camera, slightly wipe the repaired crack with my hand and take the picture. Little did I know, or realize at that moment, the powered sugar remnants on my hand would be so attracted to the plastic and show up as obviously on the crack. So, just a hint... before taking pictures of repaired cracks, wash all powered sugar from your hands. :-)