There is a heavier duty bracket option at the depot, its white and has a cross member that connects the edges of the bracket (thicker metal too). This will dramatically increase the amount of weight you can put on it. This bracket should be very near the brackets you got. Even though ...
I did stop by Home Depot about five times to wander the wood, and see what might be of use, and came upon a stock of already machined high grade Douglas Fir “rail cap” in lengths of over 12 feet. It is heavier, and very stiff, and I am tempted to bring one home and give it...
Perhaps you live in a poor area and have a bad store near you but where I live I’ve NEVER had a bad experience at a LOWES!! And I can’t say the same about Home Depot! I do shop at both weekly and do many projects and both have cut wood for me. I prefer paint from Lowe...