None that I'm aware of.
Other more salty Wby guys should weigh in here to make sure.....I'd drop these Wby guru's a PM to make sure.....TRGRMAN, JE Custom, Broz, specweldtom, Magnus.
The best option I'm aware of is the factory "378 Wby" center feed internal box which measures at approx 3.815 from front to rear. Most will load at a lesser length to ensure proper function...say 3.808 or less. This box may require some gunmithing to fit/operate properly in your action, depending if it started out as a .540 bolt face or was a 378 based case to begin with.
For bottom metal, you can order from Wby, specifically from their custom shop, a product that is called a "drop box kit". This is a one piece bottom metal, that allows for +1 of internal mag capacity with the above center feed mag, and a flat center feed mag follower. This will come with a flat mag center follower. It protrudes a little farther down than the factory metal and looks like:
That above pic is borrowed from Magnumdood at weatherby.dk
You could make the factory metal work with the Center feed box, but you might only get 2+1 on capacity with the Center feed box and factory bottom metal. Since your looking for a DBM, I'm assuming capacity is important to you.
The part numbers:
Box, Magazine Large Caliber: 6816
Drop Box Kit: CBA2038
Flat follower: I don't have the part number for that handy...sorry.
Hope that helps.