My immediate concern would be the ever-present voids you find in cork. The pen tip follows the surface, moving up and down with variations in surface relief (heights). If the pen passes over a void, the tip will drop into the void. But when it tries to move past the void it will get stuck… It needs to have a nice gentle ramp it follows up to exit the void. That is, the tip follows the surface but expects changes in height to be nice and smooth and graceful. A sudden drop isn’t necessarily a problem. But then exiting from that sudden drop is.