Simple All-Purpose Flywheel-
Here's an easy way to build a simple flywheel without creating a large circular part. Two plates are bolted together, pinching the crank between them. The outer diameter of the flywheel is formed with rubber fuel line tubing (which you could leave off if this wheel is just for testing). Although this is definitely a low speed design, decent balance can be gotten, if care is taken in assembly. Put sand in the tubing for extra mass.