Under string tension, there's a decent chance you'll crack the nut.
What's the neck width? What's the thickness of the channel? Maybe you could take a five-string blank and see if you can file the b-string end off, assuming appropriate spacing for the other strings? Just a thought.
This Graphtech Tusq part might do the trick, if you're up to filing your own string slots:
http://www.graphtech.com/products/brands/tusq/product-detail/pq-2200-00-tusq-nut-blank-flat-bottom?id=2e6ea003-39fe-434d-955b-8243b6a21687
I had prefab Graphtech Tusq guitar nut that was too thick for the nutslot and I just laid it face down on a piece of 320 sand paper and went at it until it would slide into the slot. I seem to recall from your other thread on this that you described thickness as being an issue with your nut blank. It's a pain in the neck because it's slow, but with fine paper, you won't end up knocking off too much thickness off. If you're prepared to shape a new nut from a blank, seems like you are in for a penny, so might as well, right?
Anyway, hope this helps -
Bagman