You might also want to check out these (didn't print those yet, but they might be better than my design):
For people with less space than a "standard" printing bed (220 x 220 mm or something like that).
I have a 20 x 30 cm bed, so I cut some material off left and right.
NOTE: filament tends to slide off easily when the spool is NOT in use. However, this is less likely while printing.