I’ve found a UK based supplier of cork board panels up to about 2′ x 3′ in size and in three different grades, from fine to course. I’ve yet to work out the cost but it doesn’t look extortionate if you pick the right combination of thickness and grade. I’m thinking of ordering either a medium grade board in 15mm thickness, so that it’s rigid enough for gaming but also still light enough to transport. I can then paint it to match my prototype board that I boshed together at the weekend: