Are there any conversion kits or anything to fit bigger rear strut tower speakers? It looks like there's plenty of room for bigger speakers but the box that holds them takes up all the room

 

If you are talking about an FC you can fit in 16cm speakers into the stock speaker box without much trouble. I don't know of any kits so you'd have to fabricate a top lid to fit it, as the stock box's lid is for the small speaker size. I fabricated a simple metal ring that the speaker + box bolts onto from below: