Pretty easy to fab, I made a mod for mine. The original is a single piece, a triangle with the cushion and plywood. The peak is at the junction of the two sides, fwd and the aft is toward the door. To improve storage when we replaced the cushions I hinged the triangle about halfway and the cushion is separate, it also prefers to be fit well to the lip of the sides of the berth. No reason not to make a step box level with the cabin floor to reduce the problem of leaving the elevated bed to the low floor of the vee when using the head on the next step up.
Its a pretty simple piece to make.
A simple way to make an accurate pattern is to cut some 1.5" wide strips of Masonite .
Then use a hot glue gun to attach pieces of the Masonite together.
Glue up these pieces directly in place where the missing board needs to go
Heres an example of this method.
As you can see the pattern doesn't need to look pretty at all but you'll likely get a near perfect pattern that you can then trace onto a piece of plywood.
My boat never had one, no cleats to set it on. My friend's '69 31' has the cleats and the filler cushion. If you don't have the cleats, those will also need to be added.
Another option was the filler held with an aluminum angle cushioned with a strip of fabric used for the settee cushions to protect the wood where it latched at the aft end.