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Before we figured out how to make this work we looked at a few options.

1) To trim out the window like usual and run the wood trim up the back side of the cabinet to just under the counter top. This way it looks finished from the outside but doesn't intrude on the counter top.

2) To do like a cascade effect with the counter top. So the counter top would fall over the back of the counter. We were going to carry the counter top material to the bottom as well. So basically instead of the bottom being a wood window sill it would be counter top material.

3) A take on #2, still do the cascade/waterfall effect with the counter top material on the back side of the cabinet. Then building a metal "grate", even/flush with the sink/counter top. The bottom of the window sill would be able to be cleaned because it's the same counter top material.

What I found was that wood is cheaper and easier to work with than quart, granite or other stone material. Once stained and sealed with a good sealer, as in marina grade sealer, wood is amazingly durable,

Good luck on your project!

Thank you @kday72. Currently it is just a dust collector back there and difficult to clean because i have to perch on the counter and clean painted wood!

I like the idea of putting a more durable / cleanable material on the sill (marine grade sealer sounds perfect!) so even if I have to hang over the counter it'll be a little more 'wash and wear' than a painted wood sill! I will definitely do something to the back of the cabinets as well like @lauraeds did here (https://www.houzz.com/discussions/2673562/new-reveal-long-garage-into-kitchen)

My new neighbors across the street will appreciate it anyway...