Hello Doug,
The closer the fit, the better the joint. I suspect the join between the blister and deck is not a perfect right angle and to complicate matters, the actual angle will vary along its length.
My wood-working skills are not up to the task of getting a perfect fit. Planing off the corners of the right angle along the two flat sides would help – anything which avoids a gap either between the blister and the quadrant or the deck and the quadrant will significantly help in eliminating water ingress.
I have no experience of timber steaming, but would reckon the quadrant beading would need some help (steaming?) as the radius is tight.
Good luck!
Mike