For me, the one architect that was (and remains still) light years ahead of everyone else, was Antonin Gaudi. You can find pictures of some of his parabolic arches which are round(ed). For some ideas.....Were you thinking of casting concrete, and wrapping it, or using your centering and building it old school? I like that your buddy thought "it would be cool". It would be cool....if done nicely....but you see some of the comments concerning the "shackle".....it could be an eyesore... There are probably a few ways to tackle this...I'm going to roll it around in my noggin for a bit.....