That's pretty fair shootin' for a flinter.
Can't tell a lot about the details from the photo so far as the front rest goes, but I've a suggestion for your consideration and it goes to the nature of your rests, front and back.
Aside from bearing weight and eliminating cant, the foot rest is intended to slide smoothly and consistently. This can be accomplished with a variety of materials. Commonly used surfaces range from wood to steel, the purpose being to have a very consistent and uninterrupted recoil impulse. Couple of examples here:
Low tech, built with scraps
Over the counter tech w/minor mods...maybe
Don't know that your approach with the rear bag is causing you any problems, doesn't appear so based on your groups. Having said that, the builder put the circular pad on the toe of the stock for a reason. My guess and it's nothing more than that: The pad is likely intended to rest on a smooth inclined surface for the same reasons mentioned above about foot rests. Don't know how one would contrive a back rest to match the slope of that pad, but it might serve to consider the premiss. Alternatively, it may have been intended to mate with a machine rest.
Much I don't know about slug rifles though, I could be full of hooey on the latter.
You have a nifty set of cannons there, enjoy!