This is a tough one.
Have you browsed the aero sub-forum enough to find examples of mods that pass your "good-looking" test? There are a few out there that look professional-esque to me.
3-wheeler's Insight tail comes to mind, obviously. As does
CigaR007's grille blocks. I think
a8ksh4's pickup aero shell is headed in the right direction too.
Another potential option for reshaping the front of the car, by the way, is to make a simple, "ugly" mod, then hide the construction material with a custom or modified vinyl "bra". Sewing vinyl is easier / faster than doing bodywork!
I'm in the same boat - I've been procrastinating about doing a permanent grille block for my Insight. I want it to look "pro". But rather than deciding on a fabrication approach and diving in, I've just been repeatedly re-taping the plastic sheeting that's been threatening to fall off the car for over a year now.