I have been using slime lines for my antler and shell casing pens.
They are easily customizeable and are very forgiving.
When I pull the pen apart and see that skimpy little refill I think of the the big fat advertizing or novelty pens. I hope my pens are viewed as novel but not a novelty like you wood get at a carnival midway.(Remember the leg lamp the father won on "Christmas Story"?)
Between cutting the casings roughing and turning the antler then the finishing there is a couple of hours involved in them not to mention the cost of either buying or begging materials,wear and tear on tools and shop utilities.
I need to charge for my time in those pens.( in all my work).That particular pen takes a lot more time than most, but the remarks about it fromthis group make me feel That I reall came out with a good design.
I just feel for the small increase in cost a "Parker" type refill pen is a more ,I'm not sure how to say it,quality isn't the right word,I think the buyer would be more impressed seeing a Parker type refill than the thinner "Cross" type inside the pen, that were getting more "value".
Those are my thoughts.