Added on the transient suppressing diodes. I think we should use the 1SMB15AT3G diode, which is a quick response transient suppressing zener diode at 7.25p a piece, unless we order 25 pieces (which might not be a bad idea) then it's 6.14p each. 100, it'll be 5.85p =P
Breakdown voltage is 16.7V min, and 18.5V max, well above intended supply voltage I believe. Also, the clamping is 24.4V, and the power dissipation is 600W which should be able to tolerate most things.