I successfully modified a regular paddle bit into a chamber bit as you can see in the image above. It was fairly straight forward to do it.
Here is what I did...
First I made a pattern of the shape I wanted at the bottom of the tobacco chamber. (I drew half of the shape and then folded it in half and drew the other side)
Then I cut this out and traced it onto the bit that I drew a center line onto it to line up the pattern.
Next I ground the bit to the lines I transferred to the bit. (I stopped just shy of the lines)
The final shaping was done by chucking up the bit in a hand drill and ran it in reverse. This let me use the belt sander to give me the final cutting edge on the bit that matched up on both sides.
After that I tested it on a piece of pine and then a piece of oak.
One down (3/4") two to go, 1/2" and 7/8" or 1".