The Coverking cover that Pelican sells is awesome, but expensive. Custom fit for a 914. Mine is now "worn out" after 5 years. It has some pin-holes in it that allow water in, but it was once totally waterproof.
For winter I put the Coverking on, then a waterproof tarp from the hardware store, then a cheap generic car cover on top and buckle that. The cheap cover keeps the sun exposure off of the expensive Coverking. The tarp is because the Coverking is no longer as waterproof as it once was. The Coverking works great now as a general sun and dust cover, but for the winter it is always the tarp sandwich!
One benefit of the "three-ply" cover strategy is that you'll almost always get the car wet when removing a wet cover. With the 3-ply, I take the cheap cover and roll it off inside the tarp, and the Coverking, which is basically dry, stops any water from getting into or on the car.