Have you tried the "saggestions" mod? It sits neatly out of the way on the side of the screen. When a user opens it, it opens an ajax window they can type feedback into. We have it on our site. It works great and the css can be modifie to suit your template very cheaply. It will work out of the box, do what you want it to do and works very well.

You can check it out on my site http://globaldancenetwork.com if you wish.