I think a flintlock REALLY needs a front locking device of some kind to draw it down tight. A local boy just brought me a cheap Cabela's "Hawken" flintlock rifle... that, for whatever reason, I decided to mess with... I will have to add a front bolt. A rear bolt only springs the front end up and the pan comes away from the barrel.
Since you have a Siler lock, and the frizzen spring has an outside screw, you can use a wood screw for the front end of the lock.... You just will have to remove the frizzen spring first to install and remove the lock. Not very convenient, but it would work.