Just purchased a 2012 Prius Three with sunroof and wondering if we can use thule foam blocks to carry our canoe until our thule rack system arrives?
No, to put it bluntly. Neither glass nor metal would on the roof itself would be a good idea. You don't want to put weight on part of the car that wasn't ever intended to support it. The side rails (the edges above the door) where the rack will attach is required. You'll have to wait until it arrives.
I personally wouldn't use foam blocks even on a steel roof, unless you're going a very short distance, at slow speeds. Wait for the Thule! I've used a Thule to carry a canoe for nearly 20 years, and would take it cross country. I use foam blocks only for a few miles to the local river, on my old civic.
