One of the first things I printed when I got my Robo R1 a few months ago was a good spool holder. (http://www.thingiverse.com/thing:188820)
I really like the simple way that it was designed, but it isn't perfect. I have a big spool with 2.3 kg of filament, which doesn't fit into the normal spool holder. So I recreated two pieces so it would be 50 mm wider.
Just print out these two pieces and get the rest at the original spool holder page. (see link above)
I've also included the autocad files for anyone who want to make it in there own special way.
DON'T LEAVE A HEAVY SPOOL ON IT FOR TO LONG. It's probably not strong enough to keep a heavy spool on it for to long. Just print something and take it of. 1/1,5 kg would be fine, but I don't trust my 2,3 kg spool to be hanging there. If anyone likes the wider spool holder enough, I'll probably make stronger leggs for it in the future as well. But for now: Don't be stupid, if it doesn't feel safe, don't put your spool on there.