[DiverDude]

I would consider placing the tank (on its side) on a piece of plywood. Use stretch wrap to attach the tank to the wood.

Now, if you rest the tank on the window sill (on purpose or by accident) the load is dispersed and it won't crack.

As a bonus, if you make the wood a few inches longer on each end, it gives you something to hold onto (you can never have to many places to hold with a tank that size !).