Ages since the last update - I spent way too long thinking about the floor. In the end I used a high-tech composite.. 4mm Birch plywood with 450g glass fibre on both sides. The total floor plate weighs 14kG, and you could probably get away with a bit less material than that, as it's extremely rigid. Should be quieter than a metal floor too.

Also I piped the brakes up, and improved the upper framework a bit. Not quite finished with that yet but should be ready to drive the chassis about very soon :)