I'm familiar with producing castings with expanded polyurethane foam. Most of us know it as insulation foam. In the simplest form, the process is to start with a model of the shape you want, carving it out of wood would be the obvious way to make the model, use a router to carve in some slots for pens.
Lay the model (slots up) on a stone slab, build a fence around it allowing an inch or so on all sides, and pour silicone mold material ($100+ per gallon) over the model. I won't go into using a vaccuum pump to suck air through the model or coating it and the stone slab with alachol diluted vaseline to prevent sticking. You will also need to make a lid which is a wood slap that is a little larger than the back of the model and coated with silicone mold material.
After removing the model from the mold, turn the mold over so the pen slots are down. Casting foam is formulated a little differently than insulating foam, but you could get by with the aerosol cans of insulation if you are fast. Squirt a layer of foam over the inside of the mold, before it can start foaming and rising, cover the back of the mold with the silocone coated slap and clamp it down tight. As the foam tries to rise inside the closed mold it will become denser and form a shell on the outer surfaces that are in contact with the mold and lid. You will need to spray on a layer of flocking.
Now you are ready to cast hunderds of them in the mold. But if I only needed a dozen or so of them, I think I would just make them out of wood. Here is a place that will sell you the silicone and urethane casting materials.
http://www.jgreer.com/silicone page.htm