So let's assume 21" wide and 42" tall for a mass loaded transmission line. I've done some reading on design and used the calculators here:
http://diyaudiocorner.tripod.com/formula.htm
to try to get a feel for dimensions of ported and sealed enclosures, but TL design looks quite a bit more complicated. Would you give me a starting point/idea?

The living room arrangement won't have enough room to toe in, and slanted baffles are beyond my woodworking skills for now. I'll look into supertweeters. As it is, I've found the Heathkits mostly satisfying to listen to; maybe the supertweeters can wait.