I am not sure if Harbor Freight has replacement panels available or not.
Maybe the clear corrugated roofing panels can be made to work. It would take the matching wood thingies to seal it up for sure. Thinking laying them horizontal instead of vertical might work. They don't hold up well to...
I have a harbor freight 6x8 greenhouse.
I did beef it up by attaching it to a heavy base that has long spikes driven into the ground. I also attached my wooden shelving to the framework to add rigidity.
Mine has been up a couple years and has withstood 2' of snow and several extreme wind...