First see:
http://www.kpsec.freeuk.com/components/switch.htm
You need to account for the current rating of the parts. A good amplifier can put out 30 or more amperes of current. Even a cheap 100W amplifier is capable of putting out around 3A. The hitch is that these current ratings are instantaneous, and switches are rated for continuous current, which is a lot lower: probably 1/10 to 1/100 of the maximum instantaneous current.
The switch, and any other part, needs to be capable of handling the steady current and more. Get the largest rated 2-throw switch you can. Ratshack should have one rated at 3A at 110vac, which can be good for as much as 8A at the lower voltage that most amps use (probably 56vac). It might be good enough, but see if they have one rated higher. My rules of thumb are often less than perfect. (Which is my my nick is Silicon *Burning*.) Also, use large gauge wire. You can probably get away with 16ga, or maybe even 18ga, but I'd try to find find a foot or two of 12 gauge wire.
Keep it simple and put the thing in a non-conductive case.
For this particular application, I think you can use either kind of switch: make-before-break or break-before-make. The name says it all: one switch breaks the first contact before making the other contact, the other kind makes the connection to both sides for a brief moment before breaking the first side. It's important in other applications because a make-before-break can result in a short circuit. For speakers, the difference is whether the amplifier sees 2 pairs of speakers in parallel or an open circuit. It should handle either without issues, but I'd probably choose make-before-break, since it's easier on the amp. In fact, I'd probably choose a 2-pole 3-way switch and have the middle purposely wired to both pairs, and either side wired to just one pair.
By the time you're done pricing all the parts, you may find it cheaper to buy rather than build. I'd guess it costs at least $12us to build on the cheap, unless you can find a stereo in the trash.
