Mine was built from 3 windows. Plates on the bottom with rods going all the way to the top. Chains with turnbuckles go through the Soffit into the attic and bolted to the rafters. I was told to keep an eye on it to make sure it did not go anywhere. I put the level on it and check the seal at the top every year but it has not moved. very little weight is actually on the window frame.

We also put R-26 insulation on the top and bottom of the window.