With a curved "Cheese grater" Body shop tool or coarse curved file bevel the entire perimeter of the hole to about 45-60 degrees and rough up the surrounding area about 1/2-3/4" outside the hole, this is where the matting will bite and bond. Cut many pieces of cloth to the general shape while increasing each one slightly. Tape a piece of aluminum foil to the underside of the body to cover the hole, seal the perimeter of the foil. I like to use a 1/2"-3/4" wide cheap paintbrush (buy 5 or 6). Mix the resin so it is not too "HOT" to increase the pot life and allow some time to work the mat with resin, I used cat food cans for a mixing pot. Wet the roughed up perimeter with resin and lay the first layer of cloth in and make it flat with the bottom foil. Make sure the outer edges where it contacts the roughed up body surface are free of any bubbles, lay the next layer in and duplicate, it will take about 10 layers depending on what weight cloth you are using. Thouroghly wet each layer but don't use an abundance of resin, only enough to wet all the cloth and eliminate air bubbles. Do not attempt to patch the entire thickness at one time. Scuff the cured surface of applied cloth the next day and continue.
I used this process to completly plug the hole for the filler cap and relocate it. It was un-nerving to cut a fresh 4 1/2" hole but by careful planning and following good fiber glassing practices you can make it come out very nice.
Here is a tip:
Get a short (6") piece of the same tubing diameter to replicate the brace (it could be sprinkler pipe too), determine the angle with the actual body as it passes through it (don't guess). Cut the tube to this angle and trace this shape (oblong) to a piece of pattern paper or light cardboard (like shirt box). Add some clearance to the outer dimension maybe 1/8"-3/16 all the way around, and then cut it out and tape and trace this on the body. Grommet is a piece of thin wall radiator overflow hose that is slit on one side. Plan it good it will come out nice. This is my finished product:
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