Hi Max,
I have measured mine for you.
the internal is 26cmx17.5cm, the lip is 2.5cm on the long side and 5cm on the short. The depth is 15cm closest to the bumper and 18cm nearest the wheel arch.
The box is tapered and slightly narrower at the bottom.
You may struggle to find a battery to fit a standard battery box, I have been meaning to make one slightly larger for myself.
Colin