There are many ways to mount the generator. I created a base and an adjustable cradle for the generator.
Cut a piece of 1/2" birch plywood large enough to hold the generator cradle reach the frame of the exercise bike on each side of the flywheel. Drill a hole for a 5/16" T-nut which will be used to fasten the cradle. Cut and screw 2x4's to the base on the underside the plywood.
Attach 2x4 boards to the bottom of the base using glue and screws. Lag bolt the cross piece 2x4 to the ends of the other 2x4's at the front of the base.
The two long boards need to have the ends cut to match the angle of the exercise bike frame. I chose to use a U-bolt to attach the base to the frame by drilling a series of small holes to create a slot for the U-bolt tang. An easier way would be to drill a 1/4" hole through the frame and into end of the 2x4's. You could use a lag bolt or a machine bolt into a cross dowel nut.
I chose to attach wheels to facilitate rolling the energy bike around. Because I used 1-1/8" x4 boards rather than 2x4's, I needed to add wood to support the wheels as shown in the picture.
Two of the boards need slots cut in them for the pipe section. You can drill and jigsaw slots or use a router for this task.