First, print your building. Depending on how it is constructed, you want to cut your walls horizontally. Make the cut at about 3/8 from the ground. Assemble this and you should end up with just the bottom of a building. Now assemble the upper walls and roof. Ideally, you could just stack the top part on the bottom part and access the interior that way. But a slight breeze will knock that idea to pieces. So here's my plan: print an inner "tube" to help hold the two together. The "tube" gets glued in to the top part of the building. You need to print at least part of the building so the white paper won't show through the seams. Your tube just needs to be about 3-5% smaller so it fits inside. I found it was easier to just do this by touch rather than trying to measure.
Here is a rough example I just knocked out tonight. On the left is the assembled building. 1/285 Truck for scale. On the right is the upper portion removed. If you look at the bottom of the upper portion you'll see the inner tube sticking down (I accidentally glued it in wrong side up). It might be easier instead of printing the inner tube to just use a piece of construction paper that matches the color of your building, at least a little. Lift off the top, insert troops, and replace. If necessary, leave the top off for game purposes. If I find I need to do so, I might at least color the inside with marker or somehow get rid of all that white space which would be quite distracting on the table top.
Working this out only took a minute, and only added 30 seconds to assembly (one cut, and fold the "tune" and glue it in to the upper).
