Figured I'd update this thread as I finally ordered one of the under rack bags. I went with the Held GS Tool Bag (part# 4436). It fits the 850 very well under the stock rack. I actually ended up mounting it backwards as it seemed a better fit for the shape, and it is now impossible to remove without taking off the seat. Means that no would-be-thieves will be able to get it out. I am using it as a place to store first aid equipment so in the case that I need it removing the seat to get to the bag is probably the least of my worries
Anyway, pics below for anyone considering fitting one. It's a good use of dead space.