Defender twin battery securing

Am looking to add a second battery to my defender incorporating a split charge system,watched a YouTube video showing Flatdog clamp but bit surprised couldn't refit battery box cover unless Optima batteries were used. Is this the norm or does anyone know of an alternative clamping system?

I can't clamp both with the split charger box. They're packed at the sides with bubble rap. The split charger system is in a heavily drilled plastic tub for protection.