The Harris top section is the same inside diameter as the unger no 2 section, and as groudhog said, a six inch length of Unger no 1 fits inside perfectly. I pop riveted the six inch section in place.
You then take an unger angle or cone adaptor, place it next to the six inch section and mark the section where the spring lugs are. you then drill a 6mm hole through at this point, presto your Harris will now take any Unger adaptor.
There is a way of using the Harris as it comes.
Buy a Stanley paint pad and handle and throw the pad away. This handle will make the best swivel you have ever used and fits onto the Harris cone.
On first examination, the Stanley handle swivels up and down, so would be useless, but look again and you will see a little Phillips screw, remove this and the handle will turn round, turn it 90 degrees and drill a small hole and reinsert the screw. depending on what brush you are using, you will have to cut the pad holder to fit, cut off the part of the brush that the handle screws into, and attach the pad holder with small bolts or self tappers.
You are left with a brush with a swivel that can be locked in any position, and the angle is just right for most jobs.
It also fits perfectly on the Harris pole.
And Ground hog, If you used Algarde silicone hose on the outside of your pole, you can fold it or trap it against the side of your pole, this will stop the flow any time you like, I have a small piece of hose tied to my pole with a small bungy band that girls tie their hair up with. I just fold the Algarde hose and slip it into this piece of hose.
This turns off the flow if you leave your pole.