Growing in u.k sidecupboard in loft for stealf. Problem is temps going down when lights out to ambient temp of 12 and gettin colder this will stunt groth and possibly kill them in winter.I need to find solution (maybe tiny mini heater) to raise temps to 17-20 any suggestions would be greatly received. size is 20" h 18" w 20"l.Please help save my girls befour they die of cold. p.s cand heat whole loft or move box..