I wanted my (future) ToyREP to be even smaller so I thought about hanging the filament spool above the printer. I have bought 3m of every rod so it didn't cost me a dollar. I don't know how will it hold 1kg and I wouldn't even try that. 250g is enough for me.
For 250g spool the Z-Smooth rod should be 400mm long instead of 295mm.
For 500g spool the rod should be 405mm long.
For 1kg spool the rod should be 410mm long.
Or less, or more, but you'll have to edit it :/
Files were made using these measurements:
250g Spool diameter = 154mm
500g Spool diameter = 184mm
1kg Spool diameter = 199mm
Universal inner spool diameter = 54mm
Hopefully these dimensions are all universal.
The holes (bottom ones) for 250g, 500g and 1kg spool are 6mm, 10mm and 15mm deep.
The upper rod which will be holding the filament can be of various length; mine is 225mm long and it's kinda short, so 330mm would be better. But it depends on your build.
I myself only use the 250g spool since the ToyREP ain't much of a consumer, but hopefully 500g and 1kg work too - I haven't tested them.
Don't print at high speeds or more than 0.3mm resolution.