Parts Express has a few waterproof boxes here:
http://www.partsexpress.com/webpage....ctGroup_ID=549
They have quite an extesive catalog for installation. I have used them on and off for over 15 years and have always been an excellent source. Browse around and you might find something that works for you.
I suggest using the A/B switch on you reciever if it has one and just use the volume control on it. Your speakers should be indoor/outdoor but it is not neccessary if you put some quick connectors like binding posts or bannana jack next to where the speakers mount. This way you could bring them inside when not in use. I do this for mine since no speaker that I have ever run across could handle the 60+ inches of rain that we get in the Puget Sound.
Wireless is a good idea but they do have their shortcomings. Volume levels (low) and low bass output are the biggest issues with them. If you can live with that then by all means go with wireless.