I've had a look around and I think they probably don't exist but I thought I'd check with the UKC knowledge base before moving on to alternatives! I know there are plenty options for carabiners which hold the rope captive to avoid cross loading but for a variety of reasons non of them are really ideal for what I need and I'm not looking for recomendations of said carabiners.
Petzl make the captiv bar but it's designed to fit on their carabiners (www.petzl.com/GB/en/Professional/Connectors/CAPTIV). Does anyone make something similar that aims to be universal and would fit on a standard D shaped carabiner? A stiff rubber version of the captiv that you could slide around into place is the sort of thing I had in mind but I haven't seen any.
(This is the carabiner that it would be used with - www.camp-usa.com/outdoor/product/carabiners/steel-d-pro-3lock-carabiner/)