I feel that the cabin lights currently in the boat are... shall I say very early 90ish? I was thinking of replacing them (all around) with new halogen lights - current LED lights just don't provide enough light - however I will need to do something for a back plate, maybe an oval plastic like they are in the new models.

Suggestions?

With the fall boat shows coming up, I would check some of the lighting dealers that will be displaying their stuff. I've seen some pretty nice replacement items and they can give you advice on what you may need. Take some interior shots of your boat with you so you can show them what to want to do.
Good luck.