There must be Hundreds of different Spool Holder designs out there !
This is one of my favorites:
It has scissor-arms that allow them to be adjusted for various size and width of spools.
Also no need for expensive metal bearings in this one, has good printed bearings.
Has a filament guide slot, if your set-up requires that.
It can be downloaded from thingiverse.
Click picture for larger view:
Attachment 7414
Anyone else have a Favorite ??