I did it, just made a frame with an adjustable length of aluminum tube as a sight. The tube is adjustable in 2 ways, vertical and horizontal. It does not even make the frame much more bulky, and the contraption can be disassembled quickly.
Aiming works real well, you can easily center down the length of the tube. In theory, this should deliver reproducable shots as long as you use a fixed anchor point (e.g. the corner of your mouth.
It is too dark to shoot, will do that tomorrow! Can't wait.