I just built a whirlpool system for my keggle and it works well. It's good to fill the keg with water and put a couple grains in it to test what works best, but I found if you direct the return close to the wall but not at the wall, you get a good whirlpool.
I used a 1/2 ftp to 3/8 comp coupler, a 1/2 MPT quick connect coupler, two 1/2 SS washers, two 1/2 silicon washers and a 1/2 brass nut.
If you want I can send you the McMaster Carr #'s
I then bent a length of 3/8 copper tube to make a long 90º and then put a 3/8 adapter and a 1/2 90º on the end. But did not solder it together, this way you can adjust for the depth of the pot and the angel of return.