I'm not going to be much help here but... Is this for the front or the back and what year?
Front is a pretty standard thing, and I have MB 116 there. In the tub, on my 1994 I installed Pioneer marine speakers (model ?6666 I think) Space was an issue and with the flimsy crap speaker mount LR has back there a 1/2 inch spacer was required. I intend to replace the whole speaker box some day. I understand that later Defenders, like your '97, had actual speaker boxes that stick out more. I have also seen speakers mounted on the rear of the tub and in the corner.