I started with Skip's pep model, hacked off large portions of it, used the dome from a kid's skateboarding helmet, and redefined significant portions of it along the way.
In other news, I was perusing my copy of the Art of Halo the other day and stumbled across this bit of grenade concept art:
I'm thinking I might need to bring the Thermite (middle left) and white phosphorous (bottom right) versions to life. I'm going to need more things to hang on my H3 Marines when I finally get around to doing a photoshoot.