Pick a softbox that does NOT have a specific mount, in other words it just has rods. Then pick the corresponding "speedring" to suit either your Einstein, one speedlight, or 1-2 speedlights. Take a look at
http://profoto.com/us/products/light...dring-adaptors for one example - it allows Profoto softboxes to mount on many brands of lights, though I can't tell if the AlienBees model would work on an Einstein. This way, you just change out the speedring to suit the lights you're using, and keep the same softbox.