I've been using this principle for many years because of it's simplicity and efficiency over the commercially bought specimens which were as a rule too bulky and heavy to my liking. The glass like hardness of the file provides an ability to easily crack the slag without blunting too soon.
Good welding usually does not require excessive hammering or chopping. The slimness of this tool allows access to small spots with more precision and reduce the need for unnecessary force that could leave chopping marks on your work piece.
In conclusion, it is very easy to make and cheap as dirt.
This file is approximately 6mm thick and about 2cm wide and marked out to rough proportions like on the picture.
It is then cut up, taking care to not heat up he metal too much so that the hardness of the file is retained.
A handy piece of 25 x 12 rectangular tube lying around is cut to approximately 300mm and a slot is measured out and cut to receive the hammer head. I prefer rectangular or square material that does not tend to slip and turn in your hand. In this case the dimensions of the tube makes it unnecessary to add a grip to the handle. Flat bar also works just fine.
By now, the chopping surfaces of the head are already ground... easier to do before welding.
Not too severely welded. Remember heat messes up the temper.
Ready for use
And a bit of paint for pride!