Mandolin Brothers is a NYC guitar store run by one Stan Jay, who is absolutely.. ah,let's not say obsessive; let's not say rabid..let's just say devoted... to preserving fine (and often old) guitars. His showroom of hundreds of high end guitars is climate controlled; I would give them a try to see if they have something specifically made for your purposes, if these other suggestions don't work out. If anyone has the right product, Mandolin Brothers would.

PS No one asked my views but light covers repel heat.