How can i make hollow cylinder but to be closed on top and bottom only hollow inside? I want to make a cylinder that is hollow that's simple i did: Cap Fill Type > Nothing

But then the cylinder is open from top and bottom. I want it to be closed and hollow only inside hollow or empty and not open on top and bottom.

Then i want to add a door or to cut some nice square on the cylinder side.
In the end i want to convert this to a unity3d object so i can move inside the cylinder.

The problem is how to make it hollow/empty without keep the top and bottom open?
How to cut/make door ?
How to convert to unity3d object ?

I did this simple tutorial:

Hollow Cylinder
Add a circle mesh.


After that, go into edit mode and extrude it down by selecting all the vertices, pressing E, and then dragging down on the z-axis. Then, select  all the vertices on one side of the cylinder you just made and press F to fill it in, then do the same on the other side. You can use box select to select all the vertices faster (press B and drag out).