I did the whole thing: I bought an aquarium heater for €18,50 at Holgen aquarium store in the Jacob van Lennepstraat 64, I still had a big plastic box and bought a smaller plastic box at Blokker (Ikea is better but farther away) for €4,00. I covered the big box with a cloth, the kind that does not let any light come through. A left over from the lining of my curtains. The whole installation is in a cupboard. Since teusday the temperature is at a stable 28 centrigrade!
Is there a way to post photos on this forum?
I'm a pack rat, I hardly throw away anything and as you see, sometimes it's useful. The plastic box and cloth came by handy.
Oh yeah, I discovered something. The extension cord I used looks like it's earthed on the outside but when I opened it up to be able to put the cable through a small hole I discovered it's not earthed! Only two poles, only two wires inside the white cord, the outside is fake. I bought this cord at a market for only 1 euro so what can one expect. The aquarium heater does not need earthing so it's no problem but be warned, if you need electrical equipment to be earthed, better don't use any cheap extension cord!