We had a 3 season screened porch, paver floor and a fiberglass roof when we first started over wintering tropicals.
Here's a shot taken on January 16, 2005 of a Musa 'Bordelon' banana flowering and fruiting.
I checked the weather history and we had nights of single digits and highs around 20° that January.
Susan all it takes is some clear rolled plastic (construction grade) and a stapler.
It's also called plastic film or sheeting.
At least 6 Mil. We use 8 Mil.
Plastic the windows inside and out.
Double layer outside if you can.
You are looking at under 100 dollars.
Well a heater but an oil filled one will do nicely.
Does it have a south facing set of windows for sun?
Since you have knee walls I would insulate them too.
Does the room have a concrete floor?
You can put 1" rigid insulation down on the floor then cover that w/ plastic and some of those foam interlocking playroom tiles.
What you are looking for is a room that doesn't get under about 40°.
NOT a Hot or Tropical GH.
Our Roof GH was a simple Hoophouse for the first 3 years and it did fine.
