I've had about 3 weeks to test with about 20 movies and it seems that the brace mod with gorilla glue worked. I was on vacation for the week of Thanksgiving so I missed a week but I have not heard a rattle from the sub since the mod.
HSU did recommend to use 2 braces but I was able to use one due to incorrect measurements on the brace/pieces of wood that was cut. I also added some Gorilla glue to each corner that I could see inside the sub for added stability. So far the sub sounds as tight as it did the first week I owned it. If I ever notice any rattle in the future, I'll go back and add a second brace but wanted to post my results and thank the HSU staff again for helping me.
Again, I've estimated that I had about 3000 hours of hard use on this sub or about 2 hours of use EACH day for 4 plus years so most will not notice this as I assume most folks wont hit 3000 hours of hard use until about 10 years or more.