I built a new 240w/480w, 450mm diameter heated bed from a stone floor tile, a network cable, and some furnace cement for < $20.

I gave it its own project as it hasn't burned up yet..

Arcus-3D-M1 XL Heated Bed

Minor rewiring and more furnace cement are still needed to permanently switch it to 480W as my initial resistance calculations were off. It doesn't make enough heat to be useful at 240W.

My server power supply and the bed power mosfet can handle it, but other stuff will need some changes. When I built it, I guessed I would need a < 360W bed and wiring/fuses/heat sinks were built to handle that. At that output, I also may need to drive my power mosfet harder. If it gets hot (or explodes), I'll need to add an amplifier in front of it. I'm decently above 'on', but well below 'max' for the gate voltage of that mosfet.

I also currently have zero disconnects rated for 40A, and I want it to be removable, so I'll have to figure that out.