Dough riser.

Will a 10gal bucket work? Or too small? 

http://www.cleanitsupply.com/p-1532-10-gallon-gray-round-brute-trash-can-rcp2610gra.aspx

If not, how about something like this:

http://www.stompthemgrapes.com/Buckets_and_Carboys-20_GALLON_PRIMARY_FERMENTER_and_LID.html

Cambro has some NSF certified ingredient bins that are big (but square); check their web site. 

hahaha... I bought my antique wooden dough trough for $50!  It will easily hold 50# of dough.

My grandmother used to knead large volumes on a big board on her kitchen table.  She mixed the wet ingredients with a little flour in a smaller bowl, then poured it into a flour well on the big board.  I've also read of people mixing dough right in the flour sack until it had enough consistency to take it to a board.

...approximately the same density as water, so 56 lbs of dough is approximately equivalent to 7 gals. of water.

Cambro makes food container up to 22 gal. capacity, which allows room for proofing expansion.. Here's a pic I just down loaded. It includes a sliding lid, but you can purchase them without a plain lid, or none at all. I couldn't find a price list, but I'll bet the biggest (22gal.) is a lot less than $100.00.

Let, me add I have no connection with Cambro whatever, except as a satisfied consumer.

Here's the website. http://cool.cambro.com/Food_Boxes_Storage.ashx

David G

I'd look at something that is more dual purpose, what about pizza dough proofing boxes. 

They're 18" x 26" x 3" high, you'd have to use a few, but you could reuse them for other small batch jobs.

http://www.webstaurantstore.com/cambro-db18263cw-pizza-dough-proofing-box-18-x-26-x-3/214DB18263CW.html

they fit about 20 pounds of dough, you can get lids or just use plastic wrap.