Nice setup, I was thinking of doing something similar. Although I only have 3 of the small battery trays.
I also have the gimbal rails as well as the dual battery plate like you have but for the life of me I can't figure out how one would mount two batteries on that plate as all the slots are parallel with the length of the plate. I was going to mount my EZUHF Rx on the top of the battery plate where you have your IO block and Rx. Putting the IO block on there as well is a good idea!
I was actually going to mount my ESCs under the motors on the plates provided... hmmm...
Thanks for posting the pics and sharing, I need all the help I can get