I made a couple of bobbins to fit in my PSK for fishing line and heavy duty thread.
1. I took a regular round bic pen apart and discarded everything except the tube.
2. Then I cut the tube to the length I desired with an x-acto knife.
3. After that I took an old tupperware lid and cut 2 disks for the spool ends using the x-acto knife locked into a compass to make sure both were round and the correct size. The compass point will also show you where the center of the disc is.
4. I then enlarged the hole that the compass point made so that it was slightly smaller than the hole in the end of the tube.
5. Then I took a 1/4" (IIRC) plastic screw, cut the length down to about 1/4" slathered it with Gorilla Glue, and stuck it through the disk and into the pen carcass.
Then I went through steps 4 and 5 again for the other side.
It was proabably way more of a hassle to make these spools than what it was worth to me in the end, but they worked pretty well... Especially for free...
Then I changed my psk yet again and ditched the homemade spools.