A glass top would definitely be the easiest, and probably cheapest. Just get a glass shop to cut the pieces to fit.
However, if you want a real "cover it up" canopy, that would be a bit more difficult, especially because I think corner tanks are bowed in the "front". Is that right?
If so, you'd have to bend whatever material is used for the vertical portion of the "front". I'm not sure how you'd do this with wood (I'd guess it would involve a few clamps, maybe some water, and some time). I suppose you could do it with plastic, or some thin metal, and only have it braced on the corners, or perhaps the corners and one small "post" in the middle.
Edit: by the way, All-Glass Aquarium does make a glass top (called a Versa-Top) for their 92 gallon corner bow tank. Also, Oceanic Systems (a subsidiary of AGA) makes both 90-gallon and 92-gallon corner bowfronts, and I'm sure their dealers have ready-made canopies for them available. May be pricey, though.