Last year I purchased three different types of hive top feeders. I thought all failed except for the wooden Miller top feeders that have the wooden floats.
Other's experience may differ and this is not so much a slam on those other types of feeders just my preference for one over the others based on the amount of bee drownings, ant access, and mold build up.
Anyways, what I did this winter was rip apart the feeders and covert them to a float style.
I simply ripped 3/4" white pine into 1/8" thick strips and stapled them together (using a regular staple gun). The float is one piece and fills the entire chamber.
Nothing fancy, but it floats and seems (IMO) to work better in keeping bees alive, lower the amount of pests (since bees have access to entire chamber and defend it), and thus, keeps out foreign organic material that increases the chance molding.
Here are some photos:
In this example I have a Beemax 10 frame styrene hive top feeder (which I do like for its insulating factor).
I have also made them for the brown plastic Canadian hive top feeders that you need to place a shim or shallow on top of (I would suggest never buying one of these consider how prone they are to warp and the ned for yet more equipment to use them.
In the end, baggies and jars I think are the most simple routes to go. I am making hive tops with holes cut in them to place quart jars into and then simply cap when not feeding.