Thank you , for the advice. Yes , I leave my drum out in the rain , and yes it gets water inside it . Unfortunately I have no shelter in my yard , I suppose I need to invest in a cover of sorts to keep the water out when not in use.
My father works in a metal shop , and has access to a CNC plasma table and pretty much any thickness of sheet metal. I may get him to cut me a disc with some holes in it to go on a rack just over the charcoal basket as a diffuser. It should work very much like you described , allowing drippings through but acting as a heat sink/diffuser.