Several hours later, it's still pretty solid, but has enough give in it to not be like sitting on a piece of carved granite. I might leave it at that for the moment and get it on a bike as soon as I can.
edit
It's flexing evenly along the length, doesn't feel like there's a weak point in the middle. Flaring the sides outwards also feels like its an even flex and not about to explode in to a million pieces...