A ton of sand, but that's the main issue with those systems and why it's genuinely impractical as anything but a hobbyist project. They need constant monitoring as all of those cells are from laptop and risk thermal runaway at some point. Even with the best matching possible some cell in his configuration will have higher internal resistance and create heat. "Real" large off-grid systems all use LiFePO4 and are unlikely to just catch fire. That being said from the forum post he seems well aware and he probably has individual fuse for each cell.

You could also just bury it so that the worst of the explosion is mostly mitigated. I've also seen small container setup which would probably work better than his (seemingly) wooden shed.