When I was in Singapore I found a plastic sheet in 1m, 2mm, and 3mm thickness. It sliced easily with a carton cutter knife, resisted warping (although not totally as it seems from recent experience) an holds glue and paint quite well.
I have not been able to find it anywhere else until a week ago when I took a Saturday afternoon out at the mall. It was the first outing since the start of quarantine here in Manila, so first time out in 3.5 months. While out I stopped into the National Bookstore in Glorietta Mall for some stationery. Enjoying some loiter time (there were only about 8 people in the queue behind me waiting to get in so I figured I could browse for 10 minutes at least.
I came across this PVC board shown above. BLoody brilliant stuff and cheap as chips. As I can now find this in Manila, there must be similar in other locations. Great stuff!