An easy design I am going to try this year is a migratory cover with a hole just big enough for a canning jar lid to fit into. Drill or punch some holes in the lid and invert into the hole. Jar sits atop the hive where you can see what the level is at a glance. Save the plug you cut out with the hole saw, take a few brads in the edge and drop it back in the hole when you are done. The bees will propolize it shut.
That will work Neil...I tried it, and don't do it now.
Because, sunlight hitting the jar will cause mold to grow inside the syrup. You will need to mix two to one syrup to slow that down. Or, mix clorox bleach in the syrup. Or, use essential oils to the same end, but that encourages robbing where I live.
If you want to feed from the top, an easier way is to place that jar or a bucket over the hole in the inner cover and put an empty super over that with a telescoping top. Better for the mold issue, less robbing, and no need to rely on propolization of the plug on top. That is why many experienced beekeepers prefer the way Perry described.
