Here is a link that contains many different ways of making the Bechemal Sauce.
Choose one and scale the recipe to the size you need.
http://www.foodnetwork.com/search/search-results.html?searchTerm=bechemal&form=global&_charset_=UTF-8
Or you can try the following:
http://www.cooks.com/rec/search/0,1-0,besamel_sauce,FF.html
Try out small portions of different recipes and choose the one you like best.