You could use a piece of glass, that's what most people using 3D printers have as a build surface. An easy source of glass for pen use would be a picture frame but the edges are likely sharp so be careful. Acrylic would also work and is easily obtained in small pieces from places like Lowes/Home Depot, I used Acrylic for some time on my Kossel. The plastic can stick to Acrylic very well but I had no issues using it with my printer, just test it out and see what process works if you go that route.