Gusler has written a couple articles about that type of patch box, but I can't remember if it was in Muzzle Blast or Muzzleloader mag. I have made one, and it was not that hard to make. I called Gusler to get the details on how the hinge was made on the inside. I have been attaching (pinned and then silver sordered in place) two tabs on the under side of the lid and patchbox main body, positioning the two together, cross drilling the parts, and installing one long pin that runs from side to side with about .100 of the pin sticking out on each side. I inlet the hinge portion of the patchbox with a relatively close fit from side to side and the wood on each side keeps the pin from moving out of the tabs. Wish I had some pictures, but I haven't taken any. I am going to make a patchbox this week for an early VA rifle and am in process of making a decision on using a standard type of hinge or the hidden hinge. If I do the hidden hinge, I can send you a couple of photographs of what I am using. I cut a slight bevel on the front end of the lid and matching surface of the main body allowing the front of the lid edge to slide under the main body in front of the lid, and a slight bevel in the rear of the lid and on the top side the main body allowing the lid to seat on top of the main body in the rear when closed. Sorry to confuse things a bit. I am sure that there are other ways folks are doing this, but is the way I assemble mine.
Roger Sells