Want to make a Gen slide - how thick of Alum plate?

Does the generator sit on rubber feet? If so, you don't need to provide a panel across the entire bottom.

A couple of 2x4 with 1/2" deep holes for the feet would be plenty strong enough and provide better ventilation for the generator. Just work those into the slider mechanism. (you could do something similar with angle iron/aluminum).

Check where the oil drain is, could make it easier to change the oil too if you can leave it in the tray.