Bees keep drowning in my hive-top feeder — floats, ladders, straw, or is it just the wrong feeder design?

Ontario, second year, three hives on 2:1 for the last two weeks. I'm using the 4-litre hive-top trough feeders that sit under the outer cover, and every single refill there are fifty to a hundred dead bees floating in the syrup. I put a strip of wood in as a float and it barely helped; they cling to the edges and slide in anyway. I've fed about 12 litres per hive so far and want them at weight by early October, so I'm not stopping, but this feels wasteful and slightly cruel. Things I've seen suggested: straw, wine corks, plastic canvas mesh, bubble wrap, or switching to a round rapid feeder with the cup, or an inverted pail feeder. What do you actually use, and does a syrup full of dead bees go off faster? It smells a little different by day three.