I finally took some photos as I was re-attaching some comb today to topbars and I thought you might find it helpful. There are many different ways to re-attach comb, but this one is my favorite for empty comb or light weight brood comb. Doesn't work real good with honey comb due to the weight and the bees just don't seem to jump right on those to fix the break.
Step 1. Cut the comb even across the top. Very important that it is pushed flat up against the topbar with no gap (so the bees will rapidly glue it back)
Step 2. Lay out strips of masking tape or painters tape to go completely around the comb and over the topbar
Step 3. Set the comb in position and wrap the tape up and over the comb (ending on the tops of the topbars)
Step 4. Position the comb inside the brood nest for rapid repair.
The bees will chew away the tape and drag it out. For heavier comb, it helps to "end" the tape wrap where the sides of the bar are pushed against another bar so it keeps it from slipping. I have also used skinny duct tape for honey comb repairs as it takes the bees longer to chew that away.