Spool holder for RigidBot, uses a dozen 5/16" ball bearings* (or slingshot ammo). Attach the lower screw before attaching the spool holder.
Make sure you test the strength of your print before using it. You'll want it to be strong enough that a tangled spool won't get pulled off and drop on the printer. There are two versions, the thinner one is designed to be small enough to fit the small Taulman spools, the thicker one will be stronger. I use one of each on my printer. You can use it with this filament guide to make the filament unspool consistently.
There are several internal support ribs so it should be stronger than it looks, as long as you get decent layer adhesion.