If you do not feel like buying bearings check out this model: http://www.thingiverse.com/thing:1001987
Filament spool holder / stand / roller for a very easy to obtain bearing, the 608Z ball bearing.
The space between the guides are 8mm since all my spools seem to be 7mm wide on the edge.
Initially I printed a holder that mounts on top of my printer, my printer has a glass bed. Someone on my printer support forum told me he did the same and gravity pulled a trick on him and now he had to order a new glass bed.
So I made this.
Added files with multiple parts included. Added optional screwmount with 3mm holes, can be drilled out of needed, just be careful. I would recommend only using one screwmount, that way you can use any spool size and the other one will probably keep itself in place. Please share your experiences in the comments.
Please take a picture and share if you make one ;)