Why a map?
Why not make a munch of Plasma Grenade Cup cakes? Back the cup cakes, place in the freezer to cool for about 30 minutes, mix and roll out fondant. Remove cupcakes from tray, and wrap in Fondant, gently roll around in the palm of your hand to make a ball. Add more details with colored fondant. and top off with blue colored whipped icing.
Plasma Grenade Cupcakes!
You could even use two flat cakes to make an ammo box to hold the grenades. I know publix can print custom rice paper pieces so you could print up some Halo symbols and make it look really authentic with very little effort.
My cake secret is use milk instead of water, and use 1/4 less milk than required so the cake stays firm as you work with it.
What ever you do take lots of pics!