Grab your masking tape and T pins again.

First, get the turtle deck, and make sure the front surface/edge is sanded flush.

Make a mark on the top center of the turtle deck 2 1/2" back from the front edge.

Mark the angle with a flexible ruler, giving yourself a little leeway. make sure you keep the overall length at the bottom/front, don't cut any overall length off the turtle deck.

Now using a large, flat sanding block, sand this face flat to the marks. Set it on a table and don't let it squat and spread out.

Make marks exactly 3" behind the FRONT EDGE of F4. These are the marks you will align the front bottom edges with.

The rear face should also be sanded flush, and if you did everything right, it should be either aligned with F9 or slightly forward of this point. you're good to go.

Now glue it on with polyurethane glue, using masking tape and T pins. Again leave a slight edge about 1/64" so you can sand a nice curve to it. Any more and the fuselage will get thin, any less and it'll just look blocky.

Go ahead and make the front face out of 1/8" lite ply, and the rear out of 1/8" balsa. They should be oversized, glue on with polyurethane glue (I love this stuff).