The parts are fiberglass produced in a closed mold under pressure. You can get a good repair with the other suggestions - JB Weld is mostly epoxy resin, and the fiberglass resin idea is great too but in either case you'll need something to bridge the gaps. Preferably more strands of fiberglass or pieces of glass matting. Tear the patches to shape instead of cutting so you get feathered edges.
To be as effective as possible, all traces of cracked and fractured material should be ground off, even at the risk of making the damaged area larger. You'll need so have some way of keeping things perfectly aligned while the new material sets, so if it's flat on both sides that would be helpful. Your picture shows that some of it is still able to be fitted together so you're in luck. Rather than attempting the entire repair in a single operation, I would do it in steps.
Using the area that still fits, use some JB Weld to glue it back together, fitting as tightly as you can and fixing it so that it stays in the right position while curing. After that step, you can go to work with a small disk on a die grinder and actually thin down the area that has already been glued, so you can put on a stronger layer of real fiberglass and resin without making the part thicker in the end.. The glue was just to hold it in place, not as the actual repair.
Now, you can either fill the voids with layers of fiberglass and resin, keeping the ratio of resin low so that it's as much glass as possible and then clamp flat wood on either side to keep it compressed while it cures - wax paper first will keep the wood from sticking - or you can use pre-made material such as long strand fiberglass filler in a can to do the filling. Either way, I would use the wood and clamps which will put pressure on the part to force the mix into the existing parts and fill any hidden cracks as well. Plus it will help squeeze out excess resin.
This is a lot of work but in the end you'll end up with a solid piece this way. Careful grinding and sanding to shape will make it almost invisible and it will be strong. You can make it stronger still if you use epoxy resin instead of polyester resin. Epoxy is harder to find (boat repair places or specialty fiberglass supply stores have it) and costs more but it's twice as strong and has far less smell. However it also usually requires warmer weather to work in.