If your soil is clay-y, where's the water gonna go when it fills the drain? Into the basement?
You have a couple of options. 1) Drain the water out of and away from the window well before it reaches the height of the window in the well. You'll need to keep the 'outlet' for the water to escape free and clear of the debris. Filter fabric (not landscapeing fabric) is what you can cover the opening with. (A gravel yard has this stuff.)
2) You can build a roof over-top of the well which is what I did. This way, water that would normally fill the well in a heavy rain is directed off the 'roof' and onto the surrounding soil. My well is 4' long and 18" wide. The roof I build is 5' long and 2' wide, and primed, painted, and shingled. I support it with PVC that's being held to the under-roof with copper U-bolts that hold up copper plumbing to walls and joists. I used 1" PVC and 1-1/4" U-braces. I then just pushed the PVC into the ground. They're about 18" in the ground and the roof is about 2' above the top of the aluminum well. This way, I can open the window for air AND I won't get any water in it. And if I get some wind-blown water, it isn't that much and the gravel can keep it down. Don't forget, the air between the gravel is only what's gonna be displaced by the water that enters the well. And that's not all that much water!
3) If you don't need to open the window (or it doesn't open), you can buy covers at any Home Center. I don't like the ones at the Home Centers because they're very thin, and crack easily. Try a lumber yard or a roofing/siding supply house. If neither place carries them, ask where you might get them. I got one window with a thick cover on it. It's lasted 5 years so far.