This is one we learned in girlscouts.  I've done this a few times since in other camping situations, but never in the BWCA.  You dig a small shallow pit just smaller than the base of your box, yet big enough to fill with a few hot coals.  Over the pit you place a cardboard box after removing the top and bottom and lining the inside with aluminum foil.  Half way up from the base, take a few wire coat hangers (straightened out) and run them across  as an oven rack.  You will have to poke holes on either side for the wires to stay in place.  You can use foil to cover the top or a turkey oven bag works great, too, it allows you to watch the food without letting heat out but it still withstands the heat.  Put your hot coals in the pit and a pan of whatever you want to bake on the rack and bake until its done!  This is great for any of your baked goods.  We once baked a delicious pinneapple upside down cake in the Black Hills.  The following night we used the oven again to make a homeade pizza.  And, just a few weeks ago, we baked a chocolate cake at a campground in Iowa!  But, I probably wouldn't recommend this for a BWCA trip with a lot of portaging.  Its probably not good for solo trips either.  But there's nothing like freshly baked goods when you've been away from civilization a few days!