I have substantial experience repairing plastic parts, for a living.
Based on previous conversations on this subject, the plastic used here seems to be polypropylene, or something with similar characteristics.
These plastics are very oily. Therefore, most attempts to simply glue them together fail.
It also does not "weld" well either.
Fiberglass resin, is not a good choice at all.
There are some great commercial products, for gluing these plastics. But, homeowners can't cost effectively use them, or dispense them.
I will tell you how you can do it at home, without spending a fortune.
Go to a NAPA, or other auto parts store, and buy a regular metal Fuel Tank Repair kit. (Around $14).
These kits contain all the materials you need to glue a small 2"x4" area.
You could purchase more fiberglass cloth, or use fiberglass drywall tape, and possibly stretch it out to 4"x4".
You will need additional kits if it's larger than that.
The kit should contain, a liquid 2 part epoxy, some cloth, a brush, and some emery paper.
Dry fit the pieces, and make sure they fit in the hole. Sand any edges necessary, to get them to fit.
Make sure all parts are de-greased.
Sand the back side of all pieces you want to apply glue to, with coarse paper, Aprox. 80 grit, or similar.
Using a lit propane torch, pass the flame over the areas you want to glue, several times. You do not need to heat the pieces up, they do melt at a low temperature. Just pass the flame over them at a slow to moderate rate, to burn off the oil on the surfaces.
Fit the pieces together, and use tape, on the outside, to hold them if necessary.
Mix the epoxy, as directed, and apply a thin coat on the back, in the area of the repair. Place some of the fiberglass cloth over the wet glue. Apply additional glue over the cloth, and with minimal brushing, smooth it out.
More glue and cloth can be applied over that, or in additional areas, if desired.
The glue will usually dry completely in about an hour.
Depending on how well you fit the pieces, it should look good from 10' away.
If you wanted to do a full repair, complete with painting, you would then "v" out the cracks from the outside with a grinder, pass the torch over them, and and apply glue to fill them.
When that dries, sand the area, pass the torch over it again, and apply a thin coat of filler. Sand filler, apply plastic adhesion promoter, (available at a body shop supply), and prime.
Sand entire hood using, a white Scotch Brite pad, and soft scrub cleanser. Rinse, dry, apply adhesion promoter to entire hood, and paint.
These fuel tank repair kits, are also great for fixing holes in fuel tanks.
