Made the insert. I routed 1" wide MDF, cut strip to 6" lengths, glued pieces side-by-side to closest fit in case. Covered with felt, using spray adhessive.
BTW: I picked the mdf off the cull rack, at Home Depot, for $0.56 for a quarter sheet.
This was not originally a hinged box. The hinges were the most expensive thing I added. My total cost in this box is under $10.