Hello rinamarie, what is the light orientation of your balcony? You can probably try using black drums of water so it will absorb heat during the day. But do understand drums of water will be quite heavy so try to know the weight limit of your balcony.
Also, Plumerias naturally go dormant during winter time, at times it is better to just allow it do so, and just wait for it to perk up again in late Spring. Our period of cold is not that onerous, so your unheated greenhouse will serve as a good protective area for any cold winds or rains, still keeping your plant the dryness it needs. You may have to ventilate a bit, so that not too much humidity is trapped inside. Plumerias are easily prone to black rot when it gets too cold and damp.
