Start with a box
Next, get yourself some of this stuff. This is the hard floral foam you can get in pretty much any craft store that has a fake flower department. Mine came in a pack of 6 I think, and I ended up using about 5 of them.
Measure your box, then make sure the foam fits in it as securely as you can. For this lid portion, I cut two blocks in half then used 3 of those halves, glued them together then shaved a bit more off the back since the lid is fairly shallow. Then lay out how you want everything, sketch outlines, and start carving. I used a melon baller and a butter knife for most of it. This stuff makes a HUGE mess, I had the vacuum hose handy and a small garbage can to drop chunks in.
Next, line the foam with fabric.
Trim excess fabric and glue it to the back, folding the corners down like wrapping a Christmas present, as best you can.
I added small leather straps to the lid portion to help hold everything in place.
And the whole thing closes and locks just fine. In the end, this all probably cost me less than $50.