is it really that hard to park? or do you need them to match your chrome mud flaps and wheel spinners? the skin of these cars is so thin you'd probably bend something if you did attach that silly stuff. if you adjust your rh mirror all the way down, you can probably watch your rear wheel and the curb.
This is what I do in a pinch. Honestly, the Prius is one of the easiest cars to park that I've ever owned...
...not if you asked my lady. She's convinced I'm hell-bent on killing her every time I drive. It's not my fault they put all these nifty banked curves for freeway on-ramps and off-ramps!
I tend to park very close to curbs, park and go without adjusting mirror back and forth, prius is not like other high end cars with automatic adjustment that kicks in when you shift to reverse
since the prius is kinda narrow, you don't need to risk crunching a wheel. give yourself a couple extra inches...
The best way to mount curb feelers is one curb feeler for every 4 sq/in of the body surface including the roof and the hood. Avoid glass areas. May reduce aerodynamic efficiency, but will reduce the chance of traffic collisions by making the vehicle more visible to other drivers.