[TRE Cup]

measure back a few inches from each end, where the curvature starts to deviate from the flares. Do a vertical cut there. Fasten the loose portions to the end of each flare. Set your main bumper section so its as it will be finally installed.
Now you have to make up the gap on each side. Here is how you can do this:
Form a piece of aluminum sheet to fasten to the inside of the two pieces. You are forming a bridge between the large and small parts. This will leave a gap between the fiberglass pieces

Fasten the bridge to the parts with some self tapping screws. Make sure your form follows the contours you are after.

Next- wax the heck out of the aluminum, leaving the fiberglass exposed. Scuff the fiberglass gel coat with 80 grit paper to remove the outer glossy skin. Feather this back about 3-4 inches on each side of the gap.

Get some fiberglass mat. Tear the material to make a plate of "spaghetti". Gloves, well ventilated lit area, eye protection are next. Mix up a hot mix of resin with some of the spaghetti. Using a disposable 1" wide brush, wet the joint area and start dabbing the cloth strands into the gap. Experiment with a small amount at first and let it dry between applications. Keep going till its over full and let dry. Yes, the part should be horizontal on a plastic covered bench: Gravity wins with resin.
Sand the outer layers to achieve the fit and shape you are after. Remove the screws holding the aluminum in place.
Turn the part over, sand and prep, alcohol to remove any wax residue. Fill in with more spaghetti /resin mix. Test fit, sand to shape, fill in imperfections with bondo and you have a very strong part that matches your flares