Febur used to make one, not sure if they still do. It was shaped more like the 749-999 oil cooler. Don't know specifics (how much additional face area, flow, etc.), haven't ever seen details anywhere just a listing of it on Febur's website.
You could also use the oil radiator from a 999 either in series with the standard one (this is what I've done on mine - engine is not standard though ) or on its own (so the rear cylinder doesn't get any more hot air after passing through the radiator).
You could also use other generic oil radiators such as Mocal. You only need to fabricate some holding plates and different hoses since the standard hoses won't fit to non-Ducati specific radiators.