The Frankford Arsenal #512 works pretty good with all but the longest bullets. And it's a 50 round box.
J&J products makes a perfect 100 round box for the BR.
http://www.jandjproducts.com/proddetail.php?prod=BR-100
4 boxes = 16.00but the question was directed at 100 rd boxes for 6br
I think the j and j is the best answer so far