How to fill holes with planes that follow a particular curve and how to export an .obj file?

Hi all, i am drawing the skyscraper West 57 in Manahattan for a course at the university, and thanks to your help i was able to restrict the dominion of the main surface. This is the code for the hyperbolic paraboloid with the hole:

[s,t]=meshgrid(linspace(0,1,20),linspace(0,1,20));

[X,Y,Z]=meshgrid(0:1:60,0:1:150,0:1:130);

X=(150*s);

Y=(60*t);

Z=((8+(X.*(Y)))-(8870*(s.*t)));

axis tight;

axis equal;

%generating of the rectangular hole

Z(X>=25 & X<=125 & Y>=20 & Y<=40)=nan;

surf(X,Y,Z)

hold on

And this is the figure i made with other planes on the base:

Now i need a few more tips..

How can i fill the internal part of the hole with 4 vertical planes?

And i need this figure to be rendered in softwares like 3ds max design or rhinoceros, so how can i export it into an .obj file? I have found only very difficult stuff around on the web which i am not able to understand :(

thank you for your help :)