Just an idea... but maybe something you could work with.
Take a short length of chain rated heavy enough (hardware section at Home Depot/Lowes). Run that through a precut piece of black steel pipe (the stuff they use for gas lines; plumbing section). Use some pipe insulation, rubber or foam (also, plumbing section... but I'd lean toward a rubber) and wrap this around the barbell. Then, hook some carabiners to both ends of the chain, wrap around the pipe insulation and clip to the chain.
That would be one handle.
Basically: pipe as a handle, chain connects down to the barbell and clips with carabiners, then pipe insulation to keep the chain from sliding on the barbell. The chain may still slip in the pipe.. but, you could run a bolt through the pipe and through a chain link if it's a problem.
I'm not sure if my explanation makes sense, or if this would really work for you, since this introduces some rotational instability too unlike actual farmers walk handles... but I can't see that actually being a bad thing considering that farmers walks are great for training your overall stability.
If you need it, I could throw together a diagram, or even a photo.