I installed one of the hive-top feeders I recently built (and managed to get to stop leaking!), which are based on the Miller-type feeder plans available on the beesource.com site.
I use to use those feeders and can offer a modification. If the syrup gets low enough for the bees to get under the divider, large amounts of bees will get covered in syrup and drown. Either use a float block that is thick enough to not allow them access (though I have had the floats get hung up on one end and still give them access) or place some screen over the space below the divider where the syrup flows thru.
One on the draw backs of these feeders are that the bees must find their way through the maze (up and down) to the syrup. I don't know if closing the upper entrance will help. Every day it takes them to locate it, is one less quart of syrup they could have taken. Also remember they will only feed during the day when the temperatures are warm enough for them to move about.
The best way to seal them is to melt some bees wax and pour it into a corner and then roll the feeder in a circle so that the molten wax flows across all the seams.