Are you talking about the World Models Mach II (Sky Raider)?

If so, I would add rounded formers to the top. Otherwise, the round turtle deck will not have anything to glue to due to the fact that the top of it is flat (the turtle deck area). If you don't do this, the rounded turtle deck will flatten out as it dries (using the before mentioned methods).

It shouldn't be hard to add, just cut some small formers, sort of a half circle, the width of the fuselage, and glue into place.

Are you going to modify the canopy? The original canopy may not look right with a turtle deck...

Jeff