I've thought a lot about this, and I've got a cool solution, but I'm concerned about the legality of offering advice, especially if you were to have an accident.
I think I'm alright so long as I just tell you what I would do, but even so I want to beg you to please be safe! I recommend getting professional safety advice, and I must strongly stress that I am not advising you and am merely saying what I would do/have done.
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Here's what I'd do:
Use 2 of the cheap $13 wading pools (5' diameter). Set them up side by side (just a few inches apart) on a leveled surface. Connect the wading pools with two separate 1" tubes by cutting two 1" holes, side by side, into each wading pool about 80% of the way to the top (make holes large enough for tubing but small enough to hold it snugly). One of these will carry water from pool1 to pool2, the other will return it. This circulation will be constant and will be powered by mere air.
The first tube should be shaped like a horizontal capital L, like this:
**|***|**
*
Those vertical bars in the diagram represent the edges of the wading pools (the 3 asterisks in the middle are between the pools). The 'downturn' on the far left is because you're creating an airlift: you'll run your bubbler's air hose into the bottom of that 'L' leg, where the air will rise, pushing some of pool1's water up and across the tube into pool2. When that happens, the rising water level in pool2 will instantly cause an equal amount of water to be pushed through the 2nd tube (a.k.a. the 'return tube') back into pool1, thereby creating a constant cycle powered entirely by your aerator. The 2nd tube can be totally horizontal (no need for the laying down L shape because no need to create a 2nd airlift) and would look like this:
**|***|**
I would take all the tropical plants and put them into pool2. I'd buy a $35
Allied 1000W bucket heater and safely secure it in pool1 (after they're both filled with water). To keep the pots from flooding, put them in individual waterproof plastic trash bags before setting them into the pool. I'd want the water to cover as much of the side area of the pots as possible.
I'd then set up a short plastic tent (much like you did) over these pools. Covered at night, uncovered during day (don't want to cook them, plus want them to get full light).
I would turn the bucket heater on at night and turn it off during the day, which should allow me to keep the water around 80F all the time. That 80F water would keep the roots of the potted plants at almost 80F all the time. The hottest water molecules would evaporate off the tubs as a nice warm steam that would rise up, enveloping the stems and leaves of the plants. The tent would create a sauna effect by trapping the steam.
Why 2 pools? First, I've done something like this before, and I think the bucket heater would heat up 1 pool too much if left on overnight (especially in a cramped pool with little water since the pots have displaced most of it). You could solve that by using a timer to cycle the heater on and off throughout the night, or you could add enough extra water mass that the heater just can't manage to overheat it in a night of constant use. IMO that would require 2 of these 5' wading pools (might turn out to be 3 in your environment, but I'd start with 2). Second, you may have enough plants that they won't all fit in just one pool, and may need to set some in pool1 as well as pool2.
Note that these bucket heaters are not designed for this purpose. When used long-term (weeks straight), they'll accumulate lime deposits. If those deposits are not regularly cleaned (by chiseling or dissolving in some sort of lime-away solution), the heating element can and will overheat and rupture, thereby releasing electric current directly into the water. That's extremely hazardous and has happened to me 3 times (mainly because I did a poor job of maintenance). Twice I noticed because the fish got stunned; once I got zapped myself. I would not recommend zapping oneself!!
I'd take the patio heater back for a refund and take the air heaters out.
With this method, the plants should be able to not just survive, but flourish all winter long
