I have no comment on the welding. I will let others comment on that as I have no experience.
You could bed
in some fairly high density foam with an epoxy
slurry. After that cures, shape it so that your skin fits with just a tad to spare. The wood strips seem like a good idea. Make sure they have several coats of epoxy
. Then bed
in your skin, fill any cracks with epoxy slurry and then wrap the whole thing (after proper preparation that is) with several layers of light cloth and epoxy. Fill and fair and coat with a barrier epoxy. Leave a small depression at the top around the rudder
post and fill with something like 5200.