What speakers will fit in a Boat arch

[mike@xxxxxxxxxxxxxxxxxxx]

I have a 30 foot Cruisers Rogue 3000 boat and need to get some extra
speaker coverage outside as the two small 6inch speakers aren't
handling the job.  I can only assume that the padding in the arch was
initially made for speakers.  the problem i'm having is that I have no
idea what size or where to find the speakers that will fit in this?

Here's a link to a picture of the arch

http://workoutsupplies.net/arch.jpg