Drills commonly have step-down gears between the motor and the chuck. This allows using a motor which is smaller, weaker, faster and less expensive.
Rather than have you fabricate precision metal parts, it is easier for you to buy complete drills at the store, ready to use.
Then construct several gripping jigs, using straps or belts. It is possible to drill multiple holes a couple inches from each other. All drills must be perfectly aligned together, if you don't want broken drill bits.
Also consider the duty cycle of usage. Example, my cheap electric drill (Black & Decker) heats up after a few minutes of continual use. Then it's almost too hot for my hand to grip.
As an alternative, you might mount several hand-crank drills in a row. With a lot of work you may contrive a way to drive them simultaneously from a single long rotating shaft, driven by a single motor. The gear train needs to be slow and powerful. Slow speeds are less likely to overheat drill bits. Slow speed is forgiving if you move the work as you start drilling.