An on/off switch can be just any sort of switch, or if you cant find such a thing, just connect the wires.
Most people connect the wiring this way:
Holes are drilled through pvc, bolts or screws are tapped in (airtight!), just like most sparkgaps are made.
Then wire the fan to the two bolts on the inside, and wire the battery to the same bolts, but then on the outside.
That way you have a secure and airtight way of getting those wires to your fan.
Have you already read the
spud wiki?
You should definitely check out the chamber fan and fuel meter pages, they will answer most of your questions.
Another way of mounting the fan into the chamber instead of bolting it down with brackets is by simply sanding down the fan to the right size and then squeezing it in. This is also described on the chamber fan page in the spudwiki.