There are a number of ways that you can arrange them. There is a fair amount of flexibility in placement..
Are you thinking sealed or ported? I can't remember off the top of my head if the MFW15s are better suited for ported or not.
They're not real high power, but with multiples you can do fun things.
I'm not sure what the implications are of using a very long narrow enclosure. Should work, but depending on the length - the resonant frequency might be a concern. A sono-sub build might almost be easier - if you're trying to do a long/narrow.