Mine is a 7A too.
No idea what the tray looks like so no idea if a doubler should be used.
But...
My advise. Don't mount anything to the firewall with rivets. Install nut plates. It's not fun removing a battery box with bolts let alone rivets.
As far as the battery box, look at how you will get the battery out if it swells. Try it with someone's 680. It probably wont slide up if swollen. Best thing I ever did FWF was cut the front off the Vans box and add hinges. I nused wide hinge and added about 1/4" of depth. Swapping a battery is like 2 minutes. Remove the top bracket. Pull two pins and the front comes off. Battery can be easily removed. Mine is an EarthX 680 in a Vans box. A section of baffle material takes up the extra space. A Dynon OAT probe is in the bracket right above the battery. The LED is in the bracket as well. I can look inside the oil door and see if it's not happy. Nothing I can do in flight anyway.