Manual Pigeon Releaser:

  This costs about $10 to build, less if you use what  you have lying around. 
The wood base is 1 x 6 lumber 12.5 in., glued and screwed to the upright boards. 6in high.  The wood extensions are 1 x 1 x 11.  The netting is ordinary window screen - I used it for the airflow, don't know yet how well it will hold up over time.  The area of the screen is 10 x 15.  It is folded over and attached to the metal arms with small screws with fairly large washers, hopefully to prevent tearing thru.  The metal arms are 1/2 in. aluminum - steel could be used of course. 
The metal arms are drilled and mounted to the wood with drywall screws.  The springs are 3" and are fairly light, not nearly as strong as you  see on commercial launchers but more than enough to give a good release of the bird.
The latch mechanism is where I had the most problems.  This is just one of several different ones I have on my other units.  It attaches at the bottom with a very small cabinet hinge and a metal L is used to hold down the arms.

Here it is loaded with a pigeon.