Hi Jerry - You may be able to find a cover on the ebay site that the former owner of J&R repair has active. He has a few things that are specific to HiLos available there. I've tried to find his listings with a quick search, but didn't succeed - however, I know it's there.
But, I suspect you CAN repair the one you have! From your description of the problem, yours is broken on the edges where the props connect that hold it open, and not at the piano hinge at the top. Here is a link to to the subject of plastic welding, which you can do with a simple soldering iron or gun:
You can melt a piece of HDPE (plastic) to the area that is broken, and then reinforce it with metal screen which you can drill for the mounting screws. Look through the whole video, and, there are others I've found that might seem more applicable. My plastic Folgers coffee can is made of HDPE and he uses zip ties in the video for extra plastic. You can also buy HDPE online in sheets, but I suspect you can find many things around the house that are made of HDPE.
I'm going to repair the mounting tabs on my car's bumper cover using this technique. With metal (window) screen melted in to the repaired area, it should be stronger than the original. Essentially, I'll be doing what is shown in this other video, which may be closer to what you need to do:
He uses a "plastic welder", but a soldering gun works too.