By comparison to the phobos captain, the main difference is just a cloak and random devotion stuff, like purity seals and reliquaries. If you have other marine kits, finding the drippy bits should be easy. Either separately, or snipped off other models.
Purity seals are super easy to make with green stuff and a bit of metal foil for the parchment, or just green stuff for both.
While he's in Phobos armour, the fancy schmansy big shoulder pads from standard mark 10 armour should probably fit if you need a bit of extra space for heraldry, or protective flanges.
Maybe add a tilting shield?
Maybe don't add the bolt shell charm on his gun, though, as that makes him look like an 8 year old chad gamer