Actually, I have substituted the original bellcrank mount (F14139) which bolts on the battery angles with the one installed on the RV-10 (F1035 + F1036) which I will use for a secondary battery rack. It bolts right on. No mods except that the holes left without nutplates do require to be installed then.
I've ordered the following parts from Van's
BATTERY/B.CRANK MOUNT F-1035, $25.00
BATTERY CHANNELS F-1036, $9.10
And these are the pages I have modified into my assembly manual. They come from the RV-10 manual.
Before ordering, I have done some CG calculations to see the effect of installing a 2nd battery behind the luggage compartment bulkhead. The effect is negligible especially if you install a lithium battery.