I wrote a contrafactum to rhythm changes today. Or I should say that one just occurred to the fingers of my right hand as I was playing, after working out some bass lines. I didn't do anything with the B section of it. I'm thinking that should be improvised.
I did decide to use the chords D7 / G7 / C-7 / B7 for the bridge (instead of C7 / F7), and to make a few other minor reharmonizations in the A section.