Tunnel Portals ~ What is a good way to make them?

I think I am like most of you guys, I 'could' make a tunnel portal but I am not sure it would be MGRS display quality.  But I still like to think about how they can be made.  We will need at least 7 with 5 of them being double and 2 single portals.  If they were all single portals then we will need a total of twelve.   All I am sure of right now is that the portals will be at least 12 inches high and we want the height  of the opening to be at least 10.5 inches. If you think that is to small let me know.   I know styrofoam comes in 12 inch by 4 feet long pieces that are 1 in thick.   It can easily be cut to shape and spray painted with "Krylon Short Cuts" which is made to paint styrofoam.  The advantage is that they can be produced quickly.  I am not sure if they would look good enough, they might.   I think portals look better if they are thick maybe we can double up around the opening.   

  Another way is to cut portals from plywood and again they can be painted grey and then sprayed with a "fleck" paint that makes them look like stone.  I made a wooden castle that I thought turned out good being painted that way.

  I know Carl and some of our other members could make portals out of wood.  That is a lot more work and I do not know if anyone has the time to do that.  If you have any ideas or know of techniques that would result in good looking tunnels please leave a comment.  It is one of the many things we need to model and one of the things that is fun to work on.