How can you pay them an hourly rate?
Canvasser: "I 've done 8 hours today, didn't get you any customers though as they all said no"
Window Cleaner: "Ok here's £64, see you again same time tomorrow"

You need to pay them at a fixed rate for the work they bring in. Even then I would only part with my money once I was happy it was a bona fide job.