[StevenB]

A quick and low-precision-required trick is to use some pieces of vinyl tubing. We did this our first few years with potentiometers and encoders because it doesn't require that the shafts be aligned within thousandths of an inch.

A large piece was heated up and slipped onto the large shaft, and one or two pieces of smaller tubing were used to build up the shaft on the encoder. Zip ties and fabric pins were used to secure the pieces. Obviously this method doesn't work for through-hole encoders, although I suppose you could mount them with a shaft and bearing and then connect them with tubing if you were desperate.