If the hole is small, you can use an aluminum repair epoxy. Apply a generous amount of the epoxy over the hole, smoothing it out with a putty knife or your finger. Allow the epoxy to fully cure according to the manufacturer's instructions. For larger holes, you may need to weld the area.

Place your model as usual, right click on it and add modifier>cylinder (if you want a round/oval hole). The manipulate the modifier to where you want the hole, resize/rotate etc till its right and then select it on the right, change the objects settings to no infill, top, bottom and perimeters to 0 and then slice.