This is one of those great ideas that seems so simple and yet so hard to carry out- I get 'engineer's block' with stuff like this sometimes.
My old Hornet had a bottom access for the pack and I miss that simplicity, it would really help if mfr's would standardize cell/pack size like back in the day. Not that they were much more standard, there was just a lot less choices haha. 6 cell 1200 mah Ni-Cad baby! Hump packs, the memories...
Sorry.
The simplest way to add some rigidity back to what you propose is to do a 2 level chassis, or add some bends/angles to the edges wherever material is lacking for cutouts.
I think you already knew that, but a chassis with a channel type center (Revo) would obviously be much easier to do that with. You might consider side loading with a vented box like the E-Revo if your body would allow.
the design seems entirely too complex. all you'd need is two male plugs mounted standing straight up on the chassis, soldered 90 degrees to the wire, then push the battery upside down onto the male bullets. have fixed sides and spring loaded holding tabs on both ends. done.
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It's "Dr. _paralyzed_" actually. Not like with a PhD, but Doctor like in Dr. Pepper.