Thought I'd share my winter setup.  

From the EPS forum - 13th Oct. 2019:

'I don't want to have a single moment this winter, where I'm thinking about cold nights. At the same time, I'm hoping that I can continue the growing season by providing plenty of warmth and humidity for my seedlings.

Measuring just over 2 cubic metres (160 cm x 150 cm x 95 cm) the base is constructed from 47 mm x 150 mm carcassing timber, joined with angle brackets. 
Inside the base is a sheet of 50 mm thick Ecotherm insulation board. Affixed to the board is a 24 m /120W soil warming cable. Once in situ, and with a 2 cm gap all around for drainage (the base sits on soil covered with weed control cloth), I used 2 x 40 kg bags of soft building sand to cover the cable and fill the gaps between the wooden frame and the Ecotherm. I also took the precaution of using heavy duty foil adhesive tape to seal the edges of the Ecotherm.

The sides and top are cut from 16 mm Triplewall Polycarbonate Sheet. These panels alone came to £180 inc. delivery, but were a lot cheaper than some rival companies I looked at.
The sides are joined along the length with heavy duty foil adhesive tape and the top rests in place under it's own weight. My garden is very small and sheltered, so a couple of short strips of the foil tape is plenty to stop it moving around.

Total cost: £240 (Not including digital day/night thermostat, which I already had.)

I've set the daytime temp. to 22 C and night temp. to 15 C. Let's see how long those temperatures can be maintained! I switched the soil warming cable on only last night, and fitted the thermostat today. By early this morning the air temperature at soil level was up to 23 C and with the thermostat connected, there was only 1 degree of heat loss in 3 hours between 1 pm and 4 pm. I'm sure that once the sand, pots and soil are heated thoroughly, I should be able to maintain a decent enough temperature inside the box even on the coldest days. I'm hoping for at least 14 C when the outside temp. drops to around 3 or 4 Celsius. We'll see..'

 

Heating cost is in the region of  £16 (20 USD) per month based on 24-hour usage.

With a thermostat, I estimate the current cost to be around 75% of this.

In the box:

Archontophoenix cunninghamiana
Arenga pinnata
Borassodendron machadonis
Brahea armata
Butia paraguayensis
Caryota mitis
Carpoxylon macrospermum
Ceroxylon quindiuense
Chamaerops humilis
Howea forsteriana
Livistona decipiens
Livistona alfredii
Livistona mariae
Parajubaea torallyi var. microcarpa 
Phoenix theophrasti
Rhopalostylis baueri ‘Norfolk Is.'
Rhopalostylis sapida
Sabal burmudana
Sabal minor
Syagrus romanzoffiana
Trachycarpus princeps
Verschaffeltia splendida
Washingtonia robusta

Looks great but why not do an indoor container ranch?

58 minutes ago, Rickybobby said:

Looks great but why not do an indoor container ranch?

Space is my only limitation. I live in a London apartment with two rooms totalling approx. 300 sq. ft., shared with a wife who likes to buy things and throw nothing away. 

I do have a wall-mounted container ranch in the corridor, but I think you saw that already..

Right looks good and way to go on your methods

12 hours ago, Rickybobby said:

Right looks good and way to go on your methods

Thank you.

16 hours ago, John in Andalucia said:

Space is my only limitation. I live in a London apartment with two rooms totalling approx. 300 sq. ft., shared with a wife who likes to buy things and throw nothing away. 

I do have a wall-mounted container ranch in the corridor, but I think you saw that already..

Mate, that wall mounted set up looks really good! It's a true feature in itself, a real talking point. It looks so scientific but also attractive, great idea!

10 hours ago, sandgroper said:

Mate, that wall mounted set up looks really good! It's a true feature in itself, a real talking point. It looks so scientific but also attractive, great idea!

Thanks, sandgroper! You posted a similar response back in June (I posted this photo on another topic). So two thumbs up from me!