I've been using a script like this for a long time. If I wasn't able to find it again I would have uploaded it for you, but I found it! It's an older script by Modern Algebra that allows you to set the x/y coordinates as well as the height/width of the encounter area and which enemy troops will appear in that area. Here's the address:
http://rmrk.net/index.php?topic=17124.0