That spring is a real good idea, greatly simplifies tying off the tension lines.
Your backers are thicker than the original hull, that will be the stiffest part of the boat.
Just curious, did you cut the top photo into 2" squares?
Why not slip the entire backer in end first, then pull into place with 4-5 strings, leaving it as a continuous patch?
Grind very little on the outside, keep the width to what will be covered by the speedline, the fix will then be invisible. You don't need a ton of strength on the outside, what you placed on the inside is stronger than th e original hull.
Now get that baby back in the water, those boats are a ton of fun, both solo or two up.
E C Hilliard
Nacra 5.7
Bombardier Invitation
Hilarious Tim π
Ed, the top photo shows the backers in the raw, as cast. They are made from 4" wide 9 oz. tape, 2 plys. Each ply is about 0.010" thick. Total thickness after curing is about 0.024".
Yes, they were cut in to 2" X 2" squares after curing. A single strip would have been wonderful, stronger and much faster, but the fender washer technique I used to keep the top and bottom edges aligned forced the one 2 X 2 at a time approach.
Note, I measured the hull (skin) thickness including gel coat = 0.045" by my vernier.