To keep the cat away? Maybe put a large tray or bowl filled with water in front of the opening. Or you could buy some small plants to keep in front of it so that the cat will munch on those instead. Or both ideas might work well together. I could see the cat trying to jump over the water and end up knocking a bunch of plants/lights over, which might be bad if one of the lamps fell in the water. So maybe scratch the water idea...

How about a door.:what: