Bottom line is you can't "float" it - you need to rest it on something. Countertop-thickness Granite has no strength across a span of more than a few inches.
Steel rods are often added to granite countertops to help support around dishwashers and sinks, and in those situations the granite is well supported by the cabinets itself. In your case you'd need a lot more.
If you want to do this, then what you need to do is effectively build an inset floating shelf across that opening, and then you can place the granite on that.
I would attach 2x4s level along the 3 walls using concrete anchors, and then I'd laminate together several 2 or 3 2x4s to make the front support. I'd put cross-braces the whole way (i.e., like a ladder). Obviously don't put cross supports where they will interfere with the sink, but make sure you have supports very close to the sink.
Make sure the shelf is sturdy with absolutely zero flex.
Nothing about this would be temporary.
Bottom line is nothing can flex, and the granite must not be more than a few inches from something support it (i.e., a little metal angle bracket won't be enough).