A block of wood and a 3/4 inch bullnose router bit, a router and router table. one El grande is bad enough. But three???
you may try searching for jewelry boxes that would hold large necklaces. the only other box I can think of is a bit overkill. it is the rosewood box from P.S.I. it's about 12 inches square. I use them for Pen, Letter Opener and Magnifying Glass sets.
Are you looking for a box to display the pens in? or for shipping?