You can do one of two things. build a pod that will hold a 6x9 that will fit in the tunnel and amd aim it up and towrds your head. Or what we did in our 22 SST was mount the front pair on the bulkhead firing back, I don tknow of this will work for you though.
I have built pods in the tunnels out if fiberglass but I don't know of your skill level. You could probably use some sort of prefab 6x9 box and put it down there and angle it up to you.
You should try either one of these and if you have any questions you can always ask.
stay away form the 4" coax (way too small for a boat). just adding tweeters will not sound good either.