Head: Gung-Ho modified
Rest: Viper
Helmet: dremeled Viper
When I set out to make my Mercer in Viper disguise, HypnoHustler was not only kind enough to do the dremeling for me but to dremel two Viper heads so I could choose. I chose the Red/Purple Viper for Mercer's uniform since Mercer in uniform had the red flak vest, which left me with a blue Viper with a removable helmet.
Around that same time there was a great 12" custom Cobra Soldier on Ebay that the customizer made with a bald head that just looked nice and menacing, so I copied the idea on a smaller scale.