My suggestion:
Go to your glass shop and ask for a 3 rail plasic rail and make it 3 stage instead of 2. (for the sliding top idea)
Silicone the rail to the plastic rim of the Viv to hold it in place. - That's what I do with mine.
If you think you will need to completely remove the glass top at some point then only silicone one side. It makes it much easier to remove without the glass falling in.
Don't use eggcrate. Use mosquito netting bound to a thin aluminum frame. Go to your hardware store and look for round aluminum rods. Bend it to the correct shape. You can use silicone to make a thin bead on top of the shaped rod which is now your frame. Take the mosquito netting and lay on top. Make sure it's taut. After it's dry cut away the excess. You want the aluminum frame sliding in the rails and not the mosquito netting. Don't wrap it around the aluminum rod.
***EDIT: oops I forgot to mention.... that frame should be the size of the piece you originally wanted to have flip back***
Use the framed mosquito netting section on the center rail.
This way you can close it off entirely (with the top glass closed over the netting) or have it partially vented depending on how much you open the top glass.
This also allows you to slide the alum. frame and the glass top back to go into the viv. from the top.
Let us know what you decide....
