This design is hundreds, maybe thousands of years old.  It is quite efficient and once you establish a rhythm it is less tiring than you would expect.  It is valved so that you get  airflow on the push and the pull stroke.  Kind of like circular breathing. This is the design sword smiths use for all of their work, including welding during folding.   It will supply enough air to heat large cross sections of metal.  

The key is to make everything slide smoothly so that you do not have to overcome friction as well as air pressure.

The one shown in the pictures is my portable box.  It is not as well made  as the one I had in my shop previously.  But it will give you the idea.  The other one is in storage and very heavy.  I made it out of 3/4" lumber core plywood.  I just about need help to move it.  

Here is a 3-D view.  It shows you all the elements.  The piston, shaft and handle are blue.  The flap valves are red and the stop blocks are magenta.  The box itself is in green.

An end view with dimensions

A side view with dimensions

An end view of the actual box.

 

This view show some important elements.  

1)  The flap is simply tied with cord at the corners and the cord run through holes in the end   and tied off.

 

2)  There is a 3" diameter hole in the box itself and then a smaller gasketed hole in the outside block.  This is so at assembly you can adjust the outside block to the right height.  You want the shaft to travel in a straight line and not bind as it slides in and out.

 

3)  There is a stop block to keep the piston from hitting the valve.  In the drawing I show it set at the bottom but halfway up the side is better.

 

The valve can just be seen in the lower left

 

 

Another shot on the other end showing both the upper and lower flap valve

 

 

A top view showing the stop block and the piston covered with a cheap bath mat.

So how could this be better.

1)  Cut a piece of glass to sit in the bottom of the box for the piston to slide on.

 

2)  Put Formica on the sides and top to help the piston slide.

 

3) Use something really slick to gasket the piston and the shaft.  The Japanese use raccoon fur.

 

The only tricky part is sizing the piston so that when you put the wrap on it seals as it slides but is not too tight.

 

All you need to do now is cut a hole, insert the 2" pipe and seal it with furnace cement or even clay will work.  The forge building pictures show this hole clearly.