I don't remember how I got the idea to build this before, could be someone else here had the idea before, or maybe it was when we were talking about Eds tunnel he built and such? I did a search but didn't find this.
When I built my other tunnel I did not have a decent camera. Tunnel was stored in a junk shed and got ruined by rain and humidity since. I will build another soon as I get some boxes and post pictures later. Most of the stuff I mess with is too big for a small tunnel like this, but it works good for the smaller stuff.
Here's simple way to build a small wind tunnel that works decent. Almost free! Most people already have various fans, boxes should be free, not alot of duct tape used either, a little glue.
If you can find a large box, maybe like refrigerators come in, washers, dryers, etc.., cut it as long as you can use for length. Maybe your limited in length do to room size, or maybe you can use it outdoors and make it long? Using two long boxes is better if you have the room to tape them together for length.
Glue on strips of cardboard around the outside of the box to strengthen it, sides and top, bottom optional. Try to cut the strips so those tubes run longways, top to bottom, not short side to side. Either works but long is stronger.
On one end that still has a closed end cut the opening for the fan size and put the fan sucking air out of the box. This should give you better wind in the box for testing the genny. The air being sucked out makes allot less turbulent wind, blowing air in causes turbulent winds.
Depending on length find the spot you want your genny placed for testing. It needs to be in the box away from the front, prefferably not too close to the rear. Take a Razor knife and holding it at a side angle about 45degree cut a door top, side, bottom, for a door flap. Door can be any size you want, smaller and centered in the box side is better. If you will be putting the genny in from front the door size does matter much as much, if you made a long tunnel you may have to use the door to put the genny in, make it big enough.
If you held the knife at the correct angle you should have a tappered cut, the inside of the box should have a smaller opening than the outside of the door edges. This helps the door close smooth and not pop inside. Carefully bend the door outwards make a straight fold on the not cut side for the hinge. I preffer to cut the door so that the hinged side is to the rear, don't know if it matters. Once you folded the hinge, close the door and glue on cardboard strips around the cuts so that they over lap onto the box, only glue strips to the door. These strips make a stop so that the door does not pop inside the box. If made right the inside of the box should be pretty smooth still where the door was cut.
If you want a window you can cut a hole for a piece of plexy in the box or door, keep it fairy small if in the door. Duct tape the window in place as smooth as you can on the inside of the box. Also tape it on the outside of the box.
Put the genny in the box near the center or fan side. The longer the box is the better and the genny in the center would be the best, but it needs to be back into the box aways, not right at the front. You can look in the front of the box when it's running but don't block the air flow, stay a distance away so you don't turbulate the air. Hey did I make up a new word? Turbulate 
To make the wind, put a box fan at the rear opening you cut for it and suck the air out of the box. If you need more wind than one fan makes you can make a duct work funnel. Cut another box the size of two fans side by side on one end and the size of your tunnel on the other end. Cut the sides for the correct height and duct tape those together. It would be stronger if you cut the sides a little larger and folded over the ends for flaps and glue the parts back togther and taped.
Nearly everyone has box fans, and they are cheap at $10 each, a better option though is car radiator fans. If you can use them be safe, make a protective cage/screen for them, some have really sharpe blades and I'm sure they will take off fingers. Being 12VDC fans you can vary the speed by voltage. The ones I tested startup and run very slow about 2-3V and will nearly blow you over at 12V-15V. A couple of these fans and a DC speed control and you could have about any wind speed you want in reason for a small tunnel. Make a solid frame of some type if using radiator fans, box fans don't really need a frame.
You could get or make something for checking winds speeds if you want, place it inside tunnel somewhere in front of the genny but not at very front of tunnel. I never checked wind speeds on mine. I just ran mine to see if things worked but not checking at what speeds then.
If you plan ahead and make tunnel right you could fold it down and store it in a closet or under a bed etc...
I lived in an apartment once so I know room may be limited for some people, but I had a Kitchen and livingroom connected with a large opening, maybe 30' wall to wall or more? I could have put such a tunnel there for 10-15' long easy for a few hours here and there, but I wasn't into this stuff then.
