The Cubby plans call for a small cabinet at the front of the tear. I reasoned that that would be an excellent place to seam the 1/4" plywood. The cabinet frame would cover the seam.

As you can see from the picture, the ceiling skin is placed under the jig's aluminum strip and butts up against the jig 2x4. I tightened the "C" clamps until the jig butts against the spar and the skin is in place. The aluminum bar kept the jig from twisting up as the clamps were tightened.
Doing the "Jig"