How to create geometry file

hello, i have a question about creating geometry file for further use in pdetool. the area that i would like to create has tree borders. two of them are like the borders of a square (x = 100, y = 100). the third border is a bit tricy, it it's a fourier function (f(x) = a0 + a1*cos(x*w) + b1*sin(x*w) + a2*cos(2*x*w)) and all the parameters are known.

my question is how do i take those borders and create a geometry file.

thank you for your help