I just started building my Boka and just finished my 3/8" cold finger with 1/4" wound double helix
its about 9 inches long and fits inside the 2" with about a 1/4" room on each side, I planned on using a pump to recirculate water like you. which is the reason for the double helix (its actually not a double helix cause i wound the outside coil in the same direction as the inner lol... whoops)
it shouldn't suffer from the water pump issues as the water will go to the cold finger first and exit out both coils and will essentially have twice the throughput of a 1/4" pipe. posting.php?mode=reply&f=2&t=24710#
It was a nightmare to solder! I'd never soldered before (used an oxyacetylene torch the first shot and melted copper posting.php?mode=reply&f=2&t=24710#
) so i had to tear it apart and give it another go
anyway, what i did was got a piece of 3/8" and hammered one end square with two sides flat and the other two like a V shape and soldered that together to seal the end, i then drilled two holes about an inch above that for the 1/4" tubes to go inside, i used a 3/8" bolt seen in the picture to wind the inner coil and then a larger 1" pipe for the outer coil. i then inserted the two 1/4" tubes inside the drilled holes and soldered those. it sealed up great and seems to have a nice flow when air is blown through.