Spotted this in my local hardware shop today. Advertised as a "rope clutch", is does exactly the same thing as an SRD. Only cost £0.40 each, so I got a couple to play with.
Pictures below show clutch on it's own, also fitted with a piece of rope showing allowed direction of travel. This thing locks up very tightly, no slippage whatsoever. Given the cost, I think these are a bargain.