@jwz Hrm, I don't know of anything like that. Couple thoughts:

Is the pulley intended for round belts? If it's just a random hardware store v-belt pulley, there might be some extra friction to be had with a round belt one.

I wonder if the ridges on a timing belt pulley would give enough friction (without shredding the rope). I don't see any near that diameter, but we could cut ridges into a larger plain one.

I see what you mean about using an o-ring, but what about two, sized to fit side by side inside a v-belt pulley? That would leave a little groove between them that the rope could ride in.

@attoparsec No, I'm not re-engineering the entire thing to have a second wheel, come on.

How the fuck am I supposed to know what the pulley was "intended" for?

@jwz What second wheel? I only meant modifying this one, or else a replacement like you indicated openness to in the post. (And was volunteering to help with the mods if needed.)

Is the groove semicircular, such that a round belt would fit snuggly in it with full contact all the way around, or does it have tapering straight sides and a flat bottom?

@attoparsec Oh, I thought you were suggesting sandwiching the cord between two wheels. Sorry.

The groove inside the wheel is flat on the bottom. Basically it's a 50mm x 4mm disc with a pair of 54mm x 1mm disc walls on the outside.