I have a similar, but larger 7x11 greenhouse, or rather had. When the plastic started to deteriorate after 4 years, I was going to try doing my own new cover, but discovered that many of the ends of the powder-coated steel frame pieces were badly rusted and close to falling off of the plastic connectors. I only paid $99 for it on Ebay in 2012 so I tossed the whole thing and built a new one from scratch. I used PVC pipe, and 6 mil plastic made for greenhouses. It took the better part of a day and cost about $100. If your frame is still good, find the plastic on Ebay or other sites. I used clear Gorilla tape (not cheap) to attach the end wall to the side walls, and around the door. The bottom of the plastic is stapled to the raised bed wood.

If the pvc pipe is in direct contact with the poly it will destroy the uv stabilsers.