I can tell you the biggest secret to building a humidor, As I have built one my self (A Cabinet type). A rock solid outer box is the biggest must. If the box alone will not hold humidity then the spanish cedar lining only helps somewhat. Once the box is built and the hinges placed. Place a small bright flashlight inside go into a dark room and look for how snugly the lid seats on the box bottom. (Thats why quality boxes use quadrant hinges. They leave a small amount of wood between the hinge and the edge of the box.)
The cedar lining should be at cigar Humidity (72%) before being fastened into the box. You can do this by cutting it to size and putting it in a plastic bag or such with a hygrometer and humidity solution, and let it stabilize. When you put it in the box don't go crazy with the glue. Just enought to hold the pieces in, remember it all has to expand and contract as your humidity and temperature flucuate by even a few degrees. For the best seal the bottom cedar lining should protrude up into the lid 1/4 - 3/8 " . You may have to round over the edge a bit to get the lid to clear this. But for a good seal. Have a cedar lining in the lid that comes down to rest on the one from the bottom. The next box I make I am going to try and make these overlap by making the bottom one bevel inward and the top bevel out.
If you f-i-l is a good woodworker he should have not problem doing this.