I'm drawing a simple boat. I have the exterior Hull worked out, and I'd like to cut out the empty space for the interior. "Thicken" makes a mess of any narrow-spaces that intersect, so I've created a series of off-set surfaces and trimmed them up to represent the interior I'd like. I plan to make both the hull and the interior into solids and Combine->subtract to hollow out my interior. To my question: I can extend a ring of surfaces up to a compund surface above them, but I get one little error.

the interior in general

The problem spot... the diagonal lenght on of the missing triangle is about 1/4" (on an 18' boat)

I tried extending several of the surfaces individually, but that doesn't work - I get the same error in other places. I tired patching the hole (a hack for sure.. not a real solution), but that lead to new problems. Is this just how "extend Surace" works? The seam you can see between the two surfaces where the error occurs is not a real seam - this is one surface drawn between two compound lines... Any advice?