Think a bilge keel should be the first step. My next suggestion would be to add some of the ballast
to a removable fin keel, bolted to the underside of the hull. This will increase the righting moment, but not the weight
. This can be seen if you examine U Tube video
s of cruise ships and liners. They have so much superstructure
they are inherently unstable.
several freighters and always fit captive nuts into the keel or underside. These are intended for trunnion display
mounts, but have always been prepared to bolt a keel on if needed. The nuts can easily be sealed when sailing