Latching Mosfet switch Based on work at: http://www.mosaic-industries.com/embedded-systems/microcontroller-projects/raspberry-pi/on-off-power-controller
R8 allows a shorter wait period on switch hold / raspi gpio driven low or a 3.3v Zener diode can/should be used to protect GPIO pin
Mosfets are separate devices In PCB V1 the P-Channel device is TO-220 while the N-Channel is DPAK In PCB V2 both are DPAK footprint
N-Channel mosfet is not critical anything that will start conducting around 3v will do
P-Chan mosfet more important Logic Level FET is needed and should have as low of an RDS(on) at 4.5v as possible
See Code Attached!