You can do a hull extension bracket. I recently built a couple for RIBs. Build bracket/hull extension mold, build part with it's own stringers.Bolt on part with 5200 sealant then glass seams. Quick and easy. I have a slideshow on my website homepage that shows the finished products.
I did not do the rest of the rebuild so the boats are still not complete and in the water but the 730 Willard RIB has been water tested with excellent results.