I worked a bit out of sequence than plan here... I decided to work on the drip rail ahead of the TB-1 bulkhead because of access room.
Since I have marked the center line at the inside of the TB and the cut line positions (on the sides of the TB in Section 1, I used the same method to mark the 17" mark onto the inside centerline of the TB.
I took a straight edge and clamped it against the 27 1/4" mark as a stop (as shown). Note the mark is against the face of the slanting board and not the supporting board (Fig. 14 on the plan). Since the slanting board has a certain thickness, its top face would not be able to touch the 17" mark on the TB. I used a thin ruler and extended its top surface, to make sure it reaches the target. Then I clamped another straight edge to butt against the back of the slanting board, sandwiching it in place.
Resting a laser pointer on the the slanting board, I cast a laser dot on the surface of the TB. This is because there is a certain thickness between the base of the laser and the center of the beam. By measuring the distance between the laser dot and the original 17" mark, the offset can be determined. Mine turned out to be exactly 1/4". With that information, I took all the clamps off and moved the entire set up back by 1/4".
Once all the supporting straight edges were clamped in place, I laid the laser pointer against the slanting surface and cast a series of dots (~1" apart) onto the TB surface - from the 17" mark to the 27 1/4" mark. As long as the slanting board is flat against the straight edge stop AND the laser pointer is seated appropriately on the slant surface, it will cast the same cut line onto both sides of the TB surface. Thus giving you an symmetrical cut line on both sides of the TB. It is difficult to see the laser dot in this picture because the laser was running out of juice. However, you can see the cut line I marked with my fine felt tip.
After I joined the dots with a flexible straight edge, I drilled a series of holes (~1" apart) along the future cut line. Then I duct taped the forward side of the cut line and 5 minute epoxied the 1 1/4" beveled foam strip per plan. Note that I cut each strip to about 10" in length such that I can hold it in place (conform to the curvature of the cut line) while the 5 minute epoxy cures. Next I glassed the drip rail in place.
I had a tough time removing the underlying duct tape (from under the foam). I decided to remove it after I cut the TB apart. Will see how it turns out...
I traced out the TB-1 pattern on 3/8" thick low density PVC foam per plan. I was a bit short of the foam, so I made two halves and joined them with 5 minute epoxy before glassing. I also added about an inch of height to the bottom because I raised the TB by an inch. I extended the triangle patterns down to the new depth. Picture shows TB-1 being glassed and weighed down to cure.
Once cured, the edges were trimmed and cut outs were removed. Picture shows trial fitting TB-1 to the turtle back. The horizontal stick in the front was to keep the TB at the correct width according to plan.
I floxed and taped joint TB-1 to the form with 2 plies of BID on each side.
While I was at it, I made the aluminum insert out of a 2" x 1 1/4" x 1/2" block and glassed it at designated location per plan. I made sure the edge is rounded off for future glassing. The only difference is that my insert is .5" thick such that it fits flush with my raised turtle back.