From what I have read, there is an issue with digital scales not responding when small quantities of powder are added to a thrown charge that is already on the scale. Some posters have said that they could work around this by very lightly touching the pan just after each small addition of powder. Perhaps a wind enclosure,with a linkage that allows this would be a work around.
Although not cheap, the scale that comes as a part of the Chargemaster, which is available separately, seems to respond well to small added amounts of powder. One of these, in a wind box (that also housed a battery pack), with a trickler, similar to the one that I have for my balance scale, would probably do the trick......or you could just go ahead and get a Chargemaster.