I used straw for many years with good results, very few drowned. I also use a 'fitted' wooden float with 1/4 inch holes drilled throughout, fits a 5 gallon bucket. It just rests on the syrup and as bees feed, slowly goes down to the bottom.
Both work equally well for open feeding but the wooden float drowns more bees than just plain old hay or straw........... Tried those little packing peanuts once.........they 'roll' drowning bees so I went back to hay.