Very nice indeed. Before I decided to mount my sub where the spare tire sits - I planned on mounting it in the back deck. However, once I started down that path - I quickly realized the amount of work required... and opted for the easy way out
As for mounting the amp rack - I used some 90 degree angle brackets - mounted them to the amp board - and then screwed them up into the back deck. As you said - I didn't need them to support any weight - just stabilize it and keep it from moving. Your solution looks good - but if you need to do it again - consider that route. I can post a pic if needed.
Looks great!