Full on redneck repair,,, cut the bottom out of a 1 gallon milk jug, shape to fit. You will need to redo every 2-3 years.

Here is what I did for mine. It was deteriorated beyond repair. I found a scrap piece of thin aluminum and fashioned it to fit. Bought a piece of stained glass and siliconed it on back side so it wouldn't fall out and light up the door way. Yours may have a different measurements than this one but you can see how I did. It has one screw holding it on bottom center. Hope this may help someone  

https://a.co/d/03moUOuZ

This worked for our Dolphin