Pretty Simple (as long as you have a *3D printer) Feel free to scale them up and probably have to change your media at 1 gallon and above.
I used a Thing-o-matic MakerBot with a Stepstruder MK7 to print the adapter.
I'm stirring 33oz batches of Plaster of Paris to make a baking template for a plastics component of an invention that will be going up here soon as well.
I made my STL file then scaled it to 10 to get the best blend between size potential and the right amount of strength to withstand the torque from the drill.
I was going to just use a rod and tap a hole in it, or even salvage an old electric beater head, trim the whisk parts off and just make it work as a drill bit that way.
I thought: you know if I can make the head in Inventor, I can make the bit adapter in Inventor too!
So I did.
First I made it so the whole part was laying on the XY.
Upon seeing the disparity between the top and bottom I thought I was going to have to build it vertically.
10% through I was sure it wasn't going to work because of the distance I was lofting.
Much to my surprise it got handled with very little detail loss. I did have to file it a touch (in step II), but in the end I was quite happy that I let the job keep running.
NOTE: The 'Plaster of Paris' was way more viscous than I thought. It was actually like concrete. I'm sure the head will hold up to this, but the drill was far too weak to support it. So, being a true martyr, the bit went ahead and broke off to save the drill's life. In short "Make your bit out of metal if you're working with anything of a higher viscosity than oil based paint. So since you want this to last you should probably do that anyway. I'll leave the bit STL up though.