OK.... STICK A FORK IN ME 'CAUSE
I AM DONE!!!!
FIRST a moral (lesson learned) from this rather lengthy story. If I were to do this modification again, the one thing I would do differently is.... DRILL the holes in the external plate AFTER you have drilled through your bed platform to add more "through bolt points" that way all of your screws will line up through the platform AND the inside L-Bracket without issue!! (Don't ask me WHY I know this)
I shared pictures and was working in my garage as much as I could without my rig because I can't leave it parked in my driveway that long or else the City Code Enforcement AND my silly HOA get all Hot & Bothered about it. Suffice it to say, I did the Passenger (curb) Side first and well.. it worked but the Driver (road) Side came out MUCH BETTER...I sometimes can be STUPID but I learn fast!
I added painters tape outside before using the L bracket inside as a "drill from in to out template" in hopes it would prevent any Fiberglass dissection or splitting..
it DID, or I was lucky, either one! I could see there actually IS a VERY THIN layer of aluminum inside the bed sandwich at these holes. I don't know if it carries through the entire platform or was just added where the screw points are but either way, it from what I could see would NOT be thick enough to provide ANY sort of added support to the screws.
Here was my tape outside, and a resulting "tape removed" after drilling look;
I cleaned the fiberglass best I could so that the rubber gasket material I added to the backside of my new plate's would have a nice surface and good seal/contact. Even still, for peace of mind I added a high quality flexible silicone caulk inside each drilled out hole so that I could hopefully block any water on the off-chance it made it past the rubber seal...
NOW here is where my "lesson learned" came in, the holes in the plates I fashioned and drilled over the last week did NOT match/line up with the internal L-bracket holes once they were installed from outside in. I didn't drill the holes with extra play room intentionally, I wanted as much steel behind them as possible. I apparently didn't drill perpendicular enough through the bed platform and as a result, when I tried to get the L-bracket in place things were NOT working out for me. Before anyone asks or recommends, I discovered this PRIOR to oozing that clear sealant into place through a dry-fitting test so AT LEAST I had that going for me!
SO that is the lesson learned, Prep/Clean/AND DRILL the bed platform FIRST, then use the resulting holes coming out of the platform as a template of WHERE to drill your holes in the external "washer/plate" you decide to use.
Regardless of my struggles, it came out looking not too bad for my tastes. As an added layer of protection and preventative measure, I also dressed the top and side edges of my plate with that same clear sealant I showed leaving the bottom edge unsealed so any stray water CAN leak out if it shows up some how.
I don't think if i HAD elected to use thicker external steel or aluminum AND decided to bridge it over to the side rails for added support it would have looked as bad as I feared. Standing back seeing the two 3" plates sitting in the corners seems almost normal, and I think if I had gone thicker AND extended it to the side rail, I would have added at least one more through-bolt half way between the center one and the side rail with a nice plate inside too to anchor and tighten the whole shebang.
In my observation, that L-bracket is a rather POOR selected design solution and a "T-Bracket" would have made much more sense providing rigidity and tension strength across a larger more reliable anchor area in the platform. Coupling something like that with perhaps either an aluminum frame member inside the sandwich, or some sort of rail outside the skin would really be the ideal solution as far as I am concerned... then again I didn't get an Engineering Degree so......
These Opinions and Thoughts are solely the responsibility of the poster and in NO WAY OR MEANS implies or makes any effort to claim they are/were someone else's ISSUE OR PROBLEM!
MANY THANKS to everyone that offered insight, thoughts and idea's as they certainly helped me reach this HOPEFULLY FIXED FOR GOOD revision/solution to my discovery of failing L-Bracket anchor points.
NOW, onto some serious Scotch Drinking as therapeutic recovery... well at least MENTAL recovery!