I tried a new (to me) method of feeding a light top bar hive today, so I thought I'd explain it here. No pictures, as it was getting late in the day and too dark.
I found one of my nucs (at small tbh) was disturbingly light when I hefted it the other day. So today, when the temps popped up to about 41F (after weeks of 10F or so), I decided to try to get them some sugar.
What I did was to first remove the outer cover and put my ear to the top bars until I located the cluster. Then I removed the follower board to get some room to shuffle the top bars. I poured some of the sugar, (which had a small amount of water added to make it slightly 'slushy') into the back of the hive on the hive floor, so that the bees might reach it in warmer weather.
I had the sugar ready, and cut a piece of newspaper big enough to hold the sugar pile. I also prepared two 3" x 3/4" x 1/4" pieces of styrofoam. Then, I used my hive tool to pry apart the top bars at the cluster. As the opening appeared, and I could hear the bees, I quickly covered over the central part of the opening with the newspaper, to keep the heat and bees inside. Then I pushed a piece of styrofoam between the bars at each end, to keep the bars apart, maintain the space, and to keep cold air from getting in from the sides.
Then I piled the wet damp sugar on top of the newspaper, positioning as much of it as I could over the space. On top of that, I placed my insulating cloth (I normally use between wads of burlap of other cloth to insulate between top bars and the outer cover). The cloth should keep the cold out if the bees come up through the space and eat the sugar.
We'll see how it goes. It is supposed to be above freezing here for a few days. I will check on it at some point to see if the bees are coming up.