I think you absolutely could seal it. First I would seal it on the inside. You probably could do this easily with cardboard and duct tape. The bugs are also capable of coming through most window units that I have seen. If I was faced with this problem I would go buy about 3 feet of replacement window screen, cut it to fit all sides, buy enough 1/2” aluminum strip from Lowe’s, or even better aluminum angle, put my screen over the ac and attach the aluminum strip along the edges with short 1/2” sheet metal screws to hold the screen in place. I doubt seriously that you’d have very many bugs get through that.