I have tried various methods but for me isolating an entire plant is definately less fiddly than selecting individual flowers. Pick a plant with no open flowers and cover it with a fine mesh, I use net curtains (is that a British thing?) keep checking every other day or so until you have three or four fruits emerging which you know have 'selfed'. remove the mesh, Tag the fruits and pick them at their ripest, you probably know the rest! 
