I'm trying to remember how the Rush/Cox/Resinger/Walker nexus mount batteries in their stunters.  All I can remember is that it's very clever, it involves plywood or maple rails and 3D printed brackets, and it allows for good control over where the battery ends up in the space, which means a consistent center of gravity.

Can folks please post pictures of how they mount batteries, perhaps with comments of what they liked and didn't like?

I'll start out with my "just don't do this" installation.  I like it because I was able to make it work in an existing fuselage; I don't like it because it depends on the Velcro straps not giving way, there's really no vertical adjustment, and it depends on friction for fore-aft location.

I've done several variations on battery installations. They all have their own set of benefits and problems. I am not a fan of top or bottom hatches as the battery has to sustain 10+ G's and the hatch or velcro has to hold it in. Velcro gets old and lets go. Outer side mounting, same thing. I like side loading batteries and good ventilation. The down side of the side load is it is hard to achieve a nice fitting hatch. I size the compartment small enough that a few small strips of foam either side of the battery keep it from flopping around. Some of my planes have velcro for this but they are older airplanes and I'm moving away from that. The down side of using foam is the air circulation in the battery compartment is hindered. Advantage goes to velcro in that regard. 

Here is how I have a few of mine. Pretty much a copy of what Paul does for the battery mount anyway.
Hope that helps.