I had thought about this problem a bit because up until now I’ve transferred powder from the bag into jugs in a manner that left much to be desired. I would place a jug in the chamber bay of the sift and slice a 1" diameter hole in the bag so powder could flow into a jug beneath. It worked, but was not ideal.
It turns out there is a device used to penetrate bags with powder/grain/etc so contents can be released in a controlled way. Its called a “bulk bag dispenser.” I devised my own miniature version of this to better control the dispensation of powder into the sift. Below is my very simple version (which already needs some design refinement). Maybe Formlabs wants to pay me to develop this further! 
