This is a clamp for Anet A8-style spool holders. It is designed to accomodate all spool sizes, and you can customise it for your own needs.
Hey! This thing is customisable...
if you have Fusion 360 (an educational license can be picked up for free).
Open the .f3d file, and navigate in "Modify" to the "Change Parameters" button. Once clicked, the parameters should be visible as in the photo below (you may need to expand the tab)
Once there, the comments I left should be pretty self explanatory. The measures you will need:
1) widest diameter of spool holder you have (add a bit, put in "spoolMax")
2) shortest diameter of spool holder you have (minus a bit, put in "spoolMin")
3) total space on which to put the spool (divide by 4, minus a bit, put in "length")
4) diameter of the rod you'll be putting the spool onto (add a bit, put in "rodDiameter")
As you can see, this thing is for spool holders with the triangle design, and a rod at the top. Look up "Anet A8 spool holder" :-)