Here you go, it's easier than it sounds to make.
Steamed Boston Brown bread
Bread Flour 8 oz
Whole Wheat flour 4 oz.
Light Rye flour 8 oz.
Cornmeal 8 oz
Salt 3/4 oz
Baking Soda 1 1/4 oz
Raisins 8 oz.
———————————————- combine ingredients above and mix
Buttermilk. 2 lb
Mollasses 15 oz
Muffin method, mix dry add wet to dry mix only until combined don’t overmix.
Spray some pam or similar lubricant into 3 cans
Place mixture into the containers 40 ounce or close, large tomatoes juice or sweet potato can 38 oz. and cover with foil, dome the foil over the top.
Steam upright for 3 hours, at 15lb. I used a commercial steamer, when I did it at home in my American Standard 3 hours was too much, it was too dry. I’d go 2 1/2 hours with that.
ETA, I've never broken it down but there's no reason it couldn't be divided into thirds to make one.
ETTA. I should have mentioned that when filling the containers, don't go above 2/3, it will rise a bit when steaming.