Ah, I think I see what you're thinking. The plane is UNpowered, just a sailplane or slope glider, right? I had it in my head that "hovering" meant a 3D plane, and I was wondering what the point of towing a kite string to altitude was

You want to use the tow line to tether the plane in the wind, then slowly let it out while gaining altitude. Interesting idea.

Here's my suggestion: Poke around for some sailplane winch designs. You should be able to make something out of commonly available materials, and it shouldn't be much of a challenge to adapt it to your purposes. Plus, you have the added benefit of being able to convert it back and use it as a regular sailplane winch if the idea doesn't pan out, or if the wind dies down and you have an irresistable urge to fly.