This is one of the things I try to teach when talking about rooting fig cuttings. ANYTHING is a greenhouse, even a Ziploc bag on top of the frig. Generally we are looking to control temperature or humidity, or both. Anything that does that is a greenhouse.
This is a greenhouse:
Because it provides a controlled environment and provides solar heating. You can roll down the windows or open the doors to to control temperature.
Having said that, you have created a controlled environment. Now you can add things or refine that environment. Adding a blanket or bubble wrap will increase heat retention. Adding black plastic (solar), a light bulb (50 watts ?) or small heater (less than $20 at Home Depot) will provide more heat. In these, and a hundred other ways you can improve or modify you "greenhouse" to meet you goal and objective - keeping your banana alive and well under less than ideal circumstances. And you can be "cheap" in the process. Plastic pipe, even without gluing is a simple way to make a frame. HD has 1 x 2 strips 8' long for 99 cents- and with a few drywall screws, voila, a frame.