Enviros are crossing their fingers — as well as waging letter-writing campaigns and purchasing newspaper ads — in hopes that President Clinton will declare a few more national monuments before he leaves office. Top on their list is the Arctic National Wildlife Refuge in Alaska, but not far down are about half a dozen more, including a proposed Siskiyou Wild Rivers National Monument in southwestern Oregon. So far, Clinton has created 11 monuments and expanded two more to protect a total of 4.6 million acres, a heck of a lot of land.