Since I am in the US, metric sizes can be hard to come by but I did find a 5/16" solid rod that would fill the job quite nicely. I redesigned the ends and it worked perfectly after it was printed.
You may notice that the print quality of the left holder is horrendous and I'll talk about that more in a future post that details print errors I ran into and their solutions. Stay tuned!
I designed these in google sketchup and started with the .stl files of the original spool holders.
1) Get google sketchup if you don't already have it! It is free!
2) Download the stl importer and put it into your plugins folder. (Mac users click on finder and type apple+shift+g and then type: /Library/Application Support/Google SketchUp 8/SketchUp/plugins)
3) Import the stl file. You will probably notice that the part is way larger than it should be. I am not sure why this is because I have mm set as my import stl unit, but I often have to scale the part down by ~0.03948 or so depending on the part. I suspect that this is the 1cm/1ft conversion factor but I am not positive. To calculate this conversion yourself just measure a distance in sketchup and then measure the same part in an stl viewer like netfabb basic.
In netfabb you click on Extras->New Measuring and then select the point-point option on the right pallette.
In sketchup you click on the measuring tape and measure the same distance. Note for this model I have already scaled it which is why both distances are the same.
4) To scale the objects, select them all using the cursor and press 's' and then type in the scaling factor.
5) Now you have your model imported and the correct size. The trouble is that you probably have way too many extraneous lines going all across the model. You need a method for stripping away all the excess lines and a different plugin called cleanup will do this. Dont forget to install the ttlib plugin that they recommend as well. I apologize if you need to make an account to download those files, but it feels a little wrong if I hosted them separately.
6) Once you have simplified the model you can start adjusting it! For the left spool holder model I show above, I was able to minimally adjust it by simply decreasing the radius of the hole. I first created an offset of the semicircle with the 'offset' button and then used the 'push/pull' feature to fill in the whole path.
7) After making all of your adjustments, you now need to export it as an stl file. I used this plugin but I am sure there are others.
8) Double check your stl file by opening it up with netfabb. I am a huge fan of repairing all stl files using cloud.netfabb.com. There is really nothing worse than printing out something with errors in it so it is really worth it to triple check that your stl files are exactly what you think they are.
9) It is also worth checking in slic3r or cura that each layer is what you expect too. In the right spoolholder version, it took several iterations for sketchup to properly export the cone that the rod rests on. At first it just eliminated that section of the design which would have been annoying to catch earlier.