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.
Step 1: Gatherous Up All Ye Parts
You can build it then hunt for the parts that will allow you to use the printed head. I made it at TechShop SF so I have the advantage of being able to scrap hunt from leftovers that other makers have left in bins after they finished their projects.
Step 2: Print a Head ---
What ARE you waiting for?
Step 3: Shave the Puppy!
I'd say don't seal the fray marks, whatever you're mixing the first time should cover that easy.
As you can see I didn't go home, I went big. Case was closed really quickly on this file.
I hope by the end of this step your dog still has fur.
Step 4: Make a Bit for It.
Make sure if you print one, you scale it the same way you scaled the head.
In ReplicatorG it's pretty simple you just hit the scale button, type in a value and then hit scale. No more and-then!
You may want to use Ctrl-Z between deviations so you don't have to do remember the entire sequence you put in. You'll notice that putting in 10 then hitting enter and putting 11 in and hitting enter gives you 110x scale. Vs. 10 enter, ooops, Ctrl-Z, 11 enter gives you the 11x scale you intended.
Step 5: Run and Gun!
You're done, run off and stir something already. Stuff doesn't get made on computers you know?