I have frame feeders. There are grooves on the sides, and the sides are tapered down. I did two things to reduce the number of drowned Bees. One I cut a block of wood roughly the with of the feeder when it is empty. Then drilled a hole through the side of the feeder through the block, and out the other side. Then tie wraped the block in place, this prevents boulging. Then I cut a thin strip of wood that fit into the bottom of the feeder remember the sides are tapered, so I did not want it to get fetched up as the syrup levels lowered. I cut the wood thin so it floats about level witht he syrup. So if a Bee falls in they can crawl out. Seems to work there are a few dead Bees, but about 1/2 a dozen in one weeks time. When re-filling I pour the syrup in slowly, and they all walk up in front of the rising syrup level.
Having said all of that I think I will go with hive top feeders in the future for a couple of different reasons, but the frame feeders were free, so the price was right.