You can export the surf plot values into a stl file using the following file exchange
Download the file exchange or add to path.
As an example
X=[x1,x2,x3,x4,x1]
Y=[y1,y2,y3,y4,y1]
Z=[Z_increase,Z_increase,Z_increase,Z_increase,Z_increase]
surf2stl('filename.stl',X,Y,Z) %exported the surf plot to stl format