If you just want to make it a bit more weathertight but still have some light I've had some success in the past with using heavy gauge polythene sheeting (the translucent stuff people use for groundsheets or even as waterproof membrane under concrete slab here) just make a simple 2" x 2" timber frame to fit snug in each window, warp some plastic around the frame and staple onto the back side and then screw the frame into the opening, lets some light in and keeps the worst of the weather out, if the windows are big, add some cross pieces for some extra support. Can be made out of salvaged timber, go skip (dumpster) diving on construction sites! Might at least make one area weatherproof enough to keep supplies etc dry over the winter. It won't keep people out obviously, if thats an issue then boarding up with OSB is the only answer.