I just had a look at my friends RV-14 battery box -- the existing box/tray could be modified to accommodate the ETX series battery:
1. Increase the length of the bolts that attach the FF-01403A from AN3-4A to AN3-7A or 10A.
2. Add a number of AN960 washers between the FF-01403A and FF-01403 to get the spacing for the ETX right.
3. Cut off the bottom joggle lip of the FF-01403 -or- drill out the rivets holding the bottom of the FF-01403 in place, fabriate a new bottom plate with joggle appropriate for the ETX dimensions.
You will need the foam spacers from EarthX to get the side-to-side fit right.