The idea is to keep the entire loft covered with in sulation so problem is nothing to do with fire rating and everything to do with keeping an airspace round the downlight and just as importantly the transformer, a fire hood would suffice for the light but the transformer will still overheat. You can actually buy steel covers that are big enough to protect the light and transformer allowing the insulation to lay over the top, but I would say a piece of plasterboard will do the trick.