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twicx
The silicone ones, although they seem to be good solutions, they seem messy between relays and using mosfets etc.
You are likely to need an external mosfet with ANY heater solution for a bed that large because of the amount of power involved, unless you increase the voltage, e.g. connect the two silicone heaters I suggested in series and use 24V. The mosfets, connection blocks and copper traces on most controller boards are only designed to carry about 10A. I wouldn't even put 10A through the Rumba mosfets, because of a design flaw. The type of mosfet specified in the schematic (PSMN7R0-60YS, see [reprap.org]) is not designed for 5V logic drive, so its on-resistance is not guaranteed for gate drive voltages less than 10V. No professional electronics engineer would choose that mosfet for that circuit.
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twicx
. and it still leaves me with the problem of the awkward mounting holes on the aluminium plate.
Sorry, I don't understand why. Using the two silicone heaters I suggested, the edges and the centre of the aluminium plate will be clear, so no problem for mounting holes there.
As for drilling a PCB heater pad, you are safe to drill outside the area of the copper traces.But I would expect the copper traced to go quite close to the edge of the board, except where there are pre-drilled mounting holes.