Some reading on the internet convinced me that plywood was the best material for the baseboard. Light, strong and easy to put screws into. In fact there was a massive variety of types. So, I just guessed and got one huge rectangular sheet of 12mm thick board delivered to the house.
I couldn't find anyone in Glasgow to cut it to shape and so, with only humiliating DIY experiences behind me, I decided that if I took my time then I, myself, could cut the sheet to the desired teardrop shape. And, lo and behold, with a hand-saw, I did and it only took about half an hour to do so.
Eventually, it will stand on screw-in legs from Ikea but for now it sits on some Ikea trestles. It will have to be repeatedly lifted for all of the wiring that will go underneath it.
I laid down the newspapers with the layout drawn on. They fitted perfectly.