Regarding the diode, something as standard as a 1N4001 would probably do the job.
[attachment=16:Here_we_go_again.jpg]
Top: Probably would work
Bottom: Extra diode, which could be added if the top didn't work properly.
Oh, and regarding the testing of resistance of the JE-Retro, just stick a variable into your circuit, find out what you like it at and change that to a fixed (or preset pot, which is still variable but it won't get changed sitting inside the cavity.)
Sorry for the woeful diagrams, but it's rather early and I have a hangover again