This started life as an "Acrylic Lipstick & Pencil Cube" (item 854100, $9.99) from The Container Store.

It has 9 holes, 5 of which fit the Sherline end mill holders quite nicely. The other 4 have to be bored out a bit. (Hint, don't bore 'em very deep, then you can have them hold small end mills, like 1/8" shank, without resting the tiny end mills on the bottom.) It also has several smaller holes (for the "pencils", I suppose) that hold tommy bars and such very nicely.