The clip on the F301 series pens made by Zebra is unusable, in my opinion. They are too flexible and too thin to clip onto a book/large pocket(hemming makes it very thick.
I simply took paracord, melted it and wrapped it three times. To seal it up, I unscrewed the pen and used a torch (butane, not propane lulz) to melt it, and took a pair of pliers and shoved it under the space where their is a bend (A quality macro is worth about a zillion words!)
I love the 301, and I'm pretty sure you could do it on anything metal barreled (like a M301, yay for standardized testing exam papers)
paracord was spare clippings laying like all over. about 1.5 inches ( ~30mm).