Because the alpha legion extensivley uses cultists, whom are frequently represented as I.G.s on the battlefeld, how about a fusion of the chaos space marine sprues, and an imperial guard weapon team?

you will need a chaos space marine boxed set, and one imperial guard weapon team.

create a chaos marine with the powerfist from the command sprue from the chaos space marines, build the weapons team, and leave out the pointing guardsman.

Apply chaos marine, and you have a great conversion for the alpha legion.