I would have thought in theory it should work using the nowtv stick in combination with your smartphone mobile data and link the stick via a wifi hotspot connection on your phone.
Though this post of mine is not a guarantee that it will work and it will be at your own risk and cost (unless somebody on here wants to give you full reassurances that they already done it this way previously watching Sky Sports Box Office).
Don't know how much mobile data it will need mind if it does work (could be anywhere around 1.5 to 2.5 GB per hour so bear that in mind also).
Also, you don't need to purchase a NowTV Sports Pass for this event, where you need to buy the fight via Sky Sports Box Office.
Have a read of this linked help article with FAQ about how to buy the fight and other relevant information.
If it was me i would seek the answers to all your questions by opening up the FAQ link above, where there is a help support email address for Neulion who are the company that are responsible for the boxing event and all technical issues or questions should be addressed to them.
Hopefully Neulion will be able to assist you further.
Anyway Good luck and fingers crossed Neulion will say it is possible to use the nowtv stick and your mobile wifi internet connection with their Sky Sports Box Office live stream.
Let us know how you get on with Neulion and i for one will be intrested what they say.