Raising a dovetail with a cold chisel takes some courage and some special tools. I use a chisel that I reground to a more acute angle - sharp! And I use a small sledge hammer with a short handle. Clamp the barrel in leather or wood so that the slot is in the middle of the jaws. Lay the chisel in the rectangular slot you've cut, and place the chisel so that it isn't square with the slot, but is going to start cutting on the outside of the barrel's slot first. Lay the chisel into the slot so that its bottom angle is flat against the barrel's slot, and give the chisel a short hard strike. Reposition the chisel to make a similar strike on the other side of the slot, and give it a sharp solid strike. Now place the chisel in the slot to raise the centre part, the outside corners already having been started.
This way, you are moving less metal than trying to lift the whole wide of the dovetail at once.
You may have to do this several times to raise enough steel to form your lip. But if your dovetail is only 1/32" deep, it should be fairly easy to lift the steel to form the dovetail.
Filing the rectangular slot with perfectly sharp bottom corners is far more difficult the cutting the 60 degree ends of the dovetail.
Perhaps someone will do a short tutorial on this useful job....hint hint.