The only thought i would have on running a taller batter (does a shorty 3s even exist in a hard case? I know 4s does) is to find a way to ‘thread’ a 1/8 velcro strap into the servo mount and behind the rear battery brace. The strap should be long enough if there is room enough in the servo mount. The rear battery brace screws in from the bottom so you should be able to clamp the velcro between the brace the chassis to lock it in.

just a thought. I am un aware of any produced larger battery hold down