I came up with a way to hole-saw the side with the lighter hole: I cut the hole on the right side first, and screwed the "slug" onto the left side as a locator hole for the pilot bit.
I chose these speakers because of the woofers' grill shape; it just fits top and bottom because of the flats, and the speaker itself has mounting ears rather than a complete ring.
Also, note the partial ring of foam between the headlight rim and the fairing. It's foam made for filling the gap between the window sashes when using a window A/C.