I had to change the design of the elevator because the part before the hinge line was to wide, preventing the ribs from aligning as intended. This is a small error in the plans which I only spotted after I built up the tail feathers. So, I need to move the forward rib of the elevators inward, plus some extra space to allow the elevator to move around the stabilizer freely:

I fixed this by first cutting the diagonal rib and cleaning up the joint with the main spar:

Then I glued in the new rib. I didn’t want to cut the incorrect rib off first, because this would weaken the structure, allowing the ply to deform again (this rib played a crucial role in aligning the ply core in the first place).

The next step is to cut the old rib off, I stayed clear from the new rib to prevent scratching it, and used by scroll saw to ensure a straight cut:

And there it is, a slice of rib:

After gluing in the lower rib and some sanding, it looks perfect again, you almost can’t tell anything changed:

Now, the ribs do align as intended, mission accomplished!

Also, I glued in the support blocks to the back of the spar and around the hinges. I slightly slanted those blocks so they wouldn’t protrude through the covering later on:

Now, I need to repeat this exercise for the right elevator and then it’s time to move onto finishing the stabilizer!