How to add heat to sidecupboard grow when lights out?

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..

I have come across heating mats for germinating seeds that raise soil temps by 5-10 degrees that i could fit under my pots and turn on when lights out and temps drop.Anyone know if this would work. Suggestions please...

The mats could work, but i'd suggest putting a heater with a thermostat (digital). Set and Forget It

or possibly reptile heating cable,mats or heat rock?... just found zoo med 60 watt ceramic heat emmiter used to heat reptile house at night. Screws into light fitting but gives out no light. What do u think.p.s thekrackla thankx for advise. What sort of heater remember only small box and full of plants so must be tiny. (now thinking 40 or 60w ceramic heater that screws into light fixture with a dimmer attached to adjust temp)

ok still brainstorming on little heater. For now what is the box made of? Is it insulated?

office furniture 3 quarter inch mdf inside coated with mylar

May have to just move it during the cold season then. Ill sleep on some ideas and try to hit you up

thanks for replying and trying to solve my problem