Which one do you have ? Pedersoli at one time had different grades for their Kentucky rifles .
One model came with a brass patch box . The cheaper one had no patch box .
As far as how hard it would be . Well not all that hard . But it would take some time since your rifle is already finished . Since the rifle you have should also have a wide flat profile to the side of the butt , it would make things easier as you wouldn’t have a lot of profile work to make the wood lid match the curve of the stock .
Past that you would need to know how to make the sliding lid for the box and resulting cavity .
Then using a scrap piece of brass to make the end of the lid so as to fill in the cut out that would be required in your existing butt plate . Once you did that . You then would have to make a retaining catch spring . Which isn’t had to do and in fact doesn’t even need to be an actual spring as the release movement isn’t much at all .
But know this if your not careful with the work you will be doing , you could end up needing to completely refinish the rifle