What about an option like this?

oops hit post vs image!

A custom steel fabricator can hook you right up.

I used this company when I needed to support my granite countertop on my kitchen bar.

https://originalgranitebracket.com/products/hidden-l-granite-bracket?variant=1604871619

Jeff

I built a frame out of unistrut for a piece of granite installed similar to this - but I had 3 walls to attach - leaving just the front edge "unsupported" directly. You may need a small leg of steel in that front left corner or at least a diagonal brace.

For the wall, you could just do a simple ledger board to set it on, but...... that won't allow you to build what you need to support the other 2 sides.

As Joseph said - steel is your friend here.