Can I roll some lead and prop it up beneath it and in the back
Yes you can, but
First...,
Musket locks tend to benefit from wrapping the flint in a very thin piece of lead, with a hole at the back of the flint so the flint actually touches the jaw screw, bare. Even though the cock is waaaay back from the frizzen, musket locks are a tad slower than rifle locks. It's not much, but it is there. Add to that the larger, heavier cock, and the larger chunk of flint, AND add the heavier frizzen and tougher frizzen spring, and you're having to overcome a lot of resistance to the flint impact. A slow speed when the flint scrapes the face of the frizzen means less sparks or none. The lead gives the flint a tad more inertia.
So you may want to lose the leather.
On the left is a basic lead flint wrap shape, and on the right is a "fancy" one.
In a pinch if you have a musket ball and a hammer and a concrete basement floor or garage floor, you can pound one out...DON'T forget the hole for the back of the flint!
You can test by taking a small, wooden matchstick, and place this under the back of the lead wrapped flint if the lead itself doesn't help enough, to cant the front edge of the flint at a downward angle. You can make it more permanent by swapping out the match stick (IF it worked) with a strip of lead the same size and shape as the matchstick.
LD