well you just need to build a pan below the ac unit. I would use some angle aluminum, and a thin sheet of aluminum to build a tray. Most window units require some water in them to help cool the coils.
The bigger issue IMHO is airflow to keep the coils from icing up.
Given both of these problems and the amount of space a window unit takes up I am either going to use a climaright or take a standad window unit and convert it to a climateright (basically what a a climateright is anyways)