I tried the lead ball trick and did not like it. I found it necessary to take the hump down further on many of the flints. I still use the leather to hold my flints and have no problems with them.
If you get a hump backed flint (one that does not fit well in the jaws of the lock) you can knock them down with a muzzy wheel or a diamond wheel and a dremel tool. I hold the flint in a channel lock and grind the hump off with the diamond wheel in the dremel until if is flat enough to use.
If you want to make a good knapping tool maybe these pictures will help... An on line friend helped me with these photos of his tools and they work well. Take a nail and make these tools.
Put the rifle on half cock in your shooting rest. With the flint in the jaws mind you, (also lay a cloth over the pan so no sparks go down there or pieces) place the end of the trimmed nail on the nose of the flint at almost straight down. The idea is not to take large flakes off the nose of the flint but just pressure flake it. I take my wooden short starter, catch a small section of the edge of the flint and
TAP LIGHTLY the head of the nail.
Another thing you can do is take an old kitchen knife (bread and butter knife) and dull it completely on a grinder. Or make it out of some flat iron. Lay the edge of the knife on the edge of the flint and again, lightly tap the blade with the short starter. Pressure flakes are very small. Touch the edge of the flint with your finger and you can feel a sharp edge starting to form. A little goes a long way.
When a flint was worn out, I used to practice on them and try to knap an edge. It takes practice and practice. Soon you will be doing it without even thinking about it.