Cattle Panel Greenhouse - Overwintering Container Plants

I have a similar sized greenhouse I built with 2’ of pine and the rest dual wall polycarbonate that I found on Craigslist.
I find the best thing to do is get a cheap 1500w ceramic heater and a good thermostat. The thermostat on the heater is useless because it is too close to the heater and you’ll be very wasteful or fail to keep warm enough. You want it a few feet away and not in direct fan path.
Amazon has a 120v 15a external thermostat for under $20 that solved these problems but mine failed year two when temps were under 10F. Huge disaster. For this year, I built my own external thermostat with a large solid state relay. Was half the price as the last one and should last forever.

Mandeville - you can can keep blueberries, blackberries, raspberries outside - they are cold tolerant at much higher latitudes than south LA. Satsumas are cold tolerant into the higher 20s in containers. Just so you know, raspberries don’t thrive in south LA heat & humidity - don’t expect good success with them.

I use two 250w heat bulbs like these from Tractor Supply connected to a temperature switch set to come on below 45 degrees. My container plants thrive on this setup.