Wishing Well

I love all the ideas about the wishing well, but I am looking to do something different. My fiance and I are not having a shot dance, yet afraid of missing out on a chunck of change we thought about having a wishing well... We are hoping to have guests write a little note, either wishing us well or advice. However we wouldnt mind, as with a wishing well. Guests dropping money into the well. However how do you pitch the idea to guests without sounding rude? Thanks for your help