So it's not much, and I apologize for the subpar photos, but the concept is simple, cheap, and functional and I thought I would share it!
We have an uninsulated breezeway between the garage and our 1940's house (Indiana) that happens to have south-facing windows, so we put it to use and moved our two garden towers inside for the winter.
We started a crop of broccoli, spinach, red leaf lettuce, butter lettuce, radishes and carrots in the late fall while the towers were outside. Although the lack of insulation, very cold temps, and limited light slowed things way down, we were still able to produce lettuces, broccoli, and radishes all winter long and the best part is everything will be ready for a full spring harvest in March (if we don't eat it all before then). The broccoli is the most tender and sweet I've ever tasted.
The mylar foil is simply hung behind the towers to catch the window light and reflect it back to the rearward plants. It's doesn't get much easier or cheaper than this. These are cheap-o emergency blankets from a big box store that were two for $2 over Christmas (thank you M*****s and the mountain of ad inserts you send me). I turned the foil blankets into curtains in about 10 minutes with some duck tape.
The curtain rod was a scavenged piece of electrical conduit (a few bucks new) from an old rooftop TV antenna!
Here's what it looks like from inside the breezeway. Notice the mylar foil is gold on one side and chrome on the other; I oriented the chrome or mirror finish towards the towers. In April these Garden Towers get to go back outside!
I will get a morning photo with good direct sun very soon!