As the world's worst DIY person, I am pleased to tell you that this compact comfrey maker works! The resources needed is part of a pallet (I spotted a broken one at the local recycling centre), wood preservative, a drainpipe, some plastic fixtures, a plastic bottle,a jam jar and tape.
Put the device together as in the photograph and then simply stuff the drainpipe with comfrey leaves and then let the resulting liquid drop through the nozzle at the bottom. Use the liquid in your watering can diluted 20:1.
Reader, John Clarke, suggests that if you can get a bottle that will fit into the pipe, fill it with stones or water and tie a piece of string to its top. You can put it on top of the comfrey and the weights squashes / squeezes out the liquid. You pull the bottle out again with the string to reload.
This is a tidy solution for small urban spaces.
Watch: the many uses of comfrey