Chuck and Cathy,
I haven't added any screen to that cover, but I have done it to a furnace exhaust. Where we stay, there are bee problems all summer long, so having those entry points blocked is a must. Since the furnace exhaust is hot, regular screen material won't work, so we had to use aluminum. It works well, though we don't have the screen on most of the time when the furnace runs a lot (bee's aren't out yet). Since there really isn't any heat from the refer exhaust, you don't have to have aluminum screen. We have yet to have any trouble with bees in the refer, but I wouldn't be surprised if we do soon, and then we'll add it. Some neighbors have already found large nests in the refer area, so I'm sure we will soon too. I can't say I know of any trouble with the top vent, though I haven't been up to check. There should be a screen over the opening (under the cover), but it's a large mesh and wouldn't prevent any bugs from entering, but it should prevent mice. If my memory is correct, the space between the screen is probably about 1/4".
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR