To find the optimum coil thickness you can cut aluminum washers out, the same size of the coil, and measure the drag exerted on the stack of washers by the rotor turning at a fixed rpm. Change the air gap and the number of washers and you should find an optimum thickness of aluminum. You may want to make the washer the size of three coils, so that the force is constant.

a potentially more accurate and easier way to perform this test is to set the rotor up, with a shaft, and a rope coiled around the shaft with a weight on it. the weight will apply a constant torque, and the eddy current in the aluminum washers (or test coils) will resist the motion. the RPM of the rotor should very quickly stabilize at some speed.

the more efficient the coil, the slower that rpm will be. once you get to the test coils, you could use this method to find the optimal inside diameter of the coil. as you probably know, the inside diameter of the coil is of diminishing returns. adding more turns only marginally increases the volts, at a significant increase in resistance. you can also vary the outside diameter of your generator, moving the magnets closer together or further away.

the constant torque applied, shorted coil experiment i described will quickly identify the most efficient coil structure.. the rpm will be the lowest.

its ok to use a solid block of aluminum or copper of various thicknesses as well, but there are some differences between a coil of x number of turns of uniform amps, and a solid block. the results should be similar though. i would try both and see if you arrive at the same optimal thickness of copper or aluminum. you will have to change the air gap for each datapoint of course.