You added the diode correctly. D3,4 can be 1N4148, but 1N4003 etc will work. Using schotky's for all would be OK too, and would give a bit more range. Q1 has to be a PMOSFET, with a threshold voltage in the 1 to 1.5 volt range, and an Rds of .1 to .5 ohm. I used what I had, a IRLML5203TR. It has an Rds of .25 ohm at a -3 Vgs. Q2 is pretty much any PNP: 3906,2907,3702,etc. I am using a 1W 15V Zener. The resistor values aren't set in stone either, in fact they are a bit on the high side. D4 was originally a resistor which meant R1 had to be fairly large; they can be scaled down by a factor of 5 to 10 ( going lower would just waste power.) R2,3 set the turn on point for Q2, ( R2/R3 + 1)Vbe, which is meant to be about half of Vz. I used 47 uF for C1,2 but you should use whatever you determined was needed in your setup.