We were talking about a 180 hour battery (7.5 days), not a 30 day battery.

But back of the envelope: 2.7 GWh of heat in LiH at 4 MJ/kg and 820 kg/m^3 is a sphere 9m in radius. If we put a 1m layer of microporous silica insulation (conductivity ~0.03 W/mK) this gives a thermal time constant of 300 days.

Sorry I was considering a standard container ship voyage at 30 days.

A 9m sphere of 1m thick Aerogel will probably cost above $500k just for the material.

Also lithium hydride itself reacts violently with water, not sure if you want to have a 9m radius 5 million pound sphere of it on a water vessel, this is the "safely" part.