I used the images from the Horus Heresy art books for inspiration, and the conversion was dead easy after that! Some metal rod (5cms long in this case), a trimmed down Bolter, a spike off a Chaos Trophy rack, and a blade (or the most bad-ass bayonet you've ever seen!) made from plasticard.
Here's a pic of a Custodian armed with Guardian Spear and Shield:
So next up on this project, a fully painted Custodian!