We put one on each hive. A couple of days later, they both had a lot of bees in the syrup. Many had drowned, and some were flying around inside. I didn't know how they got in to the syrup. It's screened off and seemed to work well last year.
As a stop-gap measure, I put the Brushy Mountain top feeder on the 10-frame hive and filled it with syrup. They don't make that kind of feeder for 8-frame (I sure wish they did!), so I went back to the entrance feeders on top of an inner cover with a box around them.
Tuesday, I got a silicone-calk-type of tube gun and ran a bead of silicone around the screen edges, hopefully sealing the route the bees were taking into the syrup. I put one on the 8-frame hive to test it and to make sure the silicone wasn't harmful to bees.
This also shows the neat new cart I got to pull boxes and frames from the hives to the shed.