A double-insulated setup should be fine as long as the coals are on firebrick or refractory cement. If there's adequate circulation of air outside of the 30-gal combustion chamber, the metal shouldn't get hot enough to warp. This may take significant air flow. Do you have a multi-speed fan?
Also, consider putting in a baffle to increase the amount of the wood gases that are re-circulated and burned.
Also, consider putting in a baffle to increase the amount of the wood gases that are re-circulated and burned.