Sounds like you need something durable. Is it above the sink? I'm not quote sure what application to use. A picture would help a lot!

No, it is behind dining table. Solid surface would be too thick?

It's possible to get a 2cm quartz piece, which wouldn't be too thick. Maybe, again, I don't know what it looks like, sorry. If it's just a window unit with no structure below it might not be able to hold the weight of one though. If you put it in see if you can dig up some paperwork. Most bay windows will have a 'weight limit' because of course some are not meant to carry loads from anything else.

It is a window unit, no braces underneath.