i have been thinking about modding an aug a2 to make an a3 aswell, so far i have made a first try in 3d so maybe this will help http://www.shapeways.com/model/1368794/aug-a3-toprail.html?li=my-models&materialId=6
since i havent gotten my aug yet i dont know if the measurements are correct, most of this is done by eye from various images i found online.