Found this 'D'-shaped rubrail at a boat store, scrap roll of about 50', bought for $30. The top edge of the D sticks out a bit so it sits on the horizontal portion of the rail. To make it work I had to modify rivets and the rivet setting head, as there was only about 5/16" gap inside the rubrail, where some kind of plastic insert normally goes.
See pictures below. Other photos as installed are here: https://www.forum.tinboats.net/viewtopic.php?f=3&t=43158
See pictures below. Other photos as installed are here: https://www.forum.tinboats.net/viewtopic.php?f=3&t=43158