A Triangle is what I ended up with.

First a blank is cut for the front, The archway is layed out and the sides are rounded over on the Table Router. Then the archway is cut out on the Bandsaw.

The back half of the sides are cut at a 45 degree angle on the Table saw.

Step stock is made and cut into 4" sections.

A Triangle is cut for the floor of the entrance and glued to the back of the steps then clamped with a home made jig.

The back sides of the triangle will be made of Luan. Long strips are cut at a 45 degree angle to match the front on the Table Saw. Then sized to height on the Miter Saw. They are placed in a 90 degree double clamp for gluing.

To reinforce the sides a corner piece is cut of MDF and glued to the top. All edges are then smeared with Wood Putty to smooth them and mold in the corner piece.

The now one piece back side is sanded primed and painted.

Since the edges of MDF are so porous. A Oil based primer is used as a filler to smooth the surface.

After sanding a Nutmeg basecoat is brushed on. Black is sprayed onto clingwrap and dabbed on. After drying another coat of Nutmeg is brushed on and then dabbed over with the same piece of clingwrap to give it a marble look.

A pencil line is drawn across the back sides to use as a paint and nail guide before brad nailing to the entrance base.

Last London Gray is sprayed to just above the guide lines to match the entrance archway. For ease of cleaning it is to remain in two pieces.