Pens can be turned from Corian™ or other solid counter top materials.
Corian and other trademarked countertop materials are fairly hard. Corian typically comes is sheets of 1/2-inch thickness. A slimline pen can be turned from a 1/2-inch square blank if it is drilled carefully. For larger pens, two layers of the material must be glued together to achieve sufficient thickness.
Corian does not have any "give" to it and can crack when fittings are pressed in. Use a close fitting transfer punch in a twisting motion to expand the last 1/4" of the tube until the fitting is a close slip fit. It only takes a gentle twist. You may have to use a slightly larger drill bit on some tubes.
Glue, trim, and turn the blank(s) as normal, then put a drop of glue on the inside of the tube. Don't put the glue on the fitting, the glue will squeeze out onto the surface of the fitting. Insert the fitting with a twisting motion to spread the glue over the inside of the tube. Your favorite CA, or Epoxy glues can be used. Loctite is an option, but it tends to be runny.