How can I scribe a slab into a nook with three walls?

Question

I have a slab bench I want to fit into a nook like this:

Of course, none of the walls are square, and each wall is built up as it approaches the corners.

Are there any good tricks to scribing a single slab to fit a space like this, given that I can't put it into place to directly scribe any of the walls without cutting it down to leave a gap when finished?

The best I've come up with is:

1. Set a square line between the corners.
2. Place scrap wood against the squared line, then scribe Left and Right walls onto the scraps.
3. Trace the Left and Right scribes onto a piece of plywood wide enough to span the Back. Cut Left and Right ends of the plywood along scribed lines.
4. Now place the plywood against Back wall, and scribe Back contour onto it.
5. Trace the scribed lines from all three edges of this full-width scrap plywood onto the slab.

Answer 1

I think I wrote an answer some time ago that addresses this, but I cannot find the link...

The simplest way I know to get the correct geometry is to do what cabinet shops do to make and install countertops. Get some long lattice strips approx 1/4" x 2" (+/-). Create a temporary platform at the elevation you want to fit the slab to and arrange the strips along the perimeter to create a template. Hot glue them together along with a diagonal between opposite corners to insure that your template is rigid. If there are indentations along the three wall surfaces, you can scribe a cardboard sheet along each edge to match the deviations and then glue it to the lattice strips. And that's your perfect template for the opening.

Answer 2

I would probably try to fit a template made of cardboard. Once that fits then transfer the size to the slab. Take a couple pieces of cardboard, lay them in the space and tape it them together. Trim to exact fit at the height where you're going to place the slab.

Since the walls are off-square you could get into a tricky situation when having to place the slab. If the opening is narrower than the back, you won't be able to slide it in from the front, and the thickness of the slab would prevent you from angling it and lowering it in. So it could be pretty important to figure out how you would place the slab, and this might dictate that it needs to be two or more pieces. Attempting to fit the template in will tell you if you're in trouble (check if you can just slide it in from the front, keeping it horizontal)

You could also deviate from the single slab concept and put a 1-2" accent piece in the middle, if it's impossible to slide in horizontally.