How could I print a new spool holder without a spool holder?  

Re: How could I print a new spool holder without a spool holder?

Are you talking about a spool holder for the original MK3 without the MMU? (I have seen comments that the MMU is a bit fussy about the spool holders).

If we're just talking about the plain-vanilla MK3 situation, I'd say to use one (or two) cardboard boxes (or chairs, or whatever is handy and free) with a dowel, rod, or pipe running between the two supports to hold the spool. It doesn't have to be mounted on top of or immediately adjacent to the printer - but it's probably best to have it behind the printer so the tension on the filament is reasonably constant as the printhead moves from side to side.

Try it out pulling by hand before you commit to doing a print, just to make sure that your arrangement is stable and allows the spool to turn without the whole structure getting pulled apart or the rod creeping off the end of the supports.

Re: How could I print a new spool holder without a spool holder?

just put the roll in a bucket on the floor.

Re: How could I print a new spool holder without a spool holder?

Get creative?

Re: How could I print a new spool holder without a spool holder?

[...] I was wondering if anyone knew the best way I could keep the filament in place as a temporary measure as I printed a new one?
Anything that will allow the spool to turn freely will work in a pinch. So long as I'm careful not to snarl the filament, I've done just fine with long strands of loose, coiled filament. Just be around as it prints so you can hear any problems before they get serious. You could start with printing a spool holder that doesn't require as much filament. There are plenty of designs on Thingiverse.