Mike - I think the biggest advice for if you are trying to do a Line source is don't start off building a speaker. Buy a few of the drivers and experiment with just a handful of drivers in throw-away baffles to start learning. Also read EVERYTHING you can find on beaming, comb filtering and generally how emissions from drivers behave.
Everyone is so against this because these only work because you get really subtle wave interactions working, where that is a function of angle of the drivers, distance, and all the driver behaviors themselves. You really simply will not get it right on a first try, and it is not as forgiving as regular box speakers are of error, so if you are really committed, the first thing is to play around understanding how arrays of drivers interact before you start to worry about an actual speaker.
I still think that is super difficult, and I am ignoring the crossover stuff on this (all the behaviors of those waves change with frequency making it even harder), but that trial and error on a few drivers is at least a way to start.
Again, read everything you can on beaming too.