I haven't made one either, though I've been looking into it, as well. Most of what I've found (in terms of traditional Dunkels) is that they don't use any roasted grains at all. I'd be inclined to replace the chocolate malt with a bit of CaraMunich.
I'd also keep the IBU's in the 12-15 range, but that's just me.
OTOH, (to play devil's advocate with myself) I just made a Doppelbock with 1/4lb of Carafa II because I like the roastiness I find in Celebrator and that is absent from Optimator, so fortunately we, as homebrewers, aren't bound by tradition!