The HPI Firestorm E-10 shell. Wheelspin and Modelsport UK usually have them in stock.
Here is one on my macnum...
You will have to drill new clip holes in the macnum's front mounts (orig run across body) but apart from that it fits quite well...
The Pro-Line crowed pleaser 2 might also fit..well it fits the Royal Flash...so might fit the Macnum.
EDIT: The Pro-line fits the macnum very well.. I'd say it's actually probably slightly a better design and fit for the Macnum then the Firestorm.
J