The length of the Sapele plywood sheet forced me to add 5" tip to make the entire deck plate. Instead of going through the pain of scarf jig setup to do three inches worth of joinery, I decided to butt join it.
The first step here was to match the wood grain of the main plate with a scrap of additional material. The scrap piece already had a clean and straight edge which served as a cutting 'straightedge'. Next, I used a sharp knife to slice through the surface veneer to get a corresponding clean cut.

The scored 1/8" plywood is easy to snap.

The backing block is made from the same material but I feathered it under the assumption that it would be covered with fiberglass. Since the butt joint ended up to be only 1/2" wide I removed (scraped out) the block immediately prior to inside deck sanding and glassing. Had the seem been in a more structurally sensitive area the block would be left in. Either way, the block helped to hold the tip in place until all plates were solidly glassed on the outside.