The point you raise about the other water is why I'm revamping the design, hoping to get one of these neat new drip trays. 95% of the water is caught by the PVC in the bottom of the cage and goes down the funnel no problem. The other 5% runs off on the outside of the screen, getting under the cage and causing problems - the water that doesn't sit under the cage gets into the cheapo ikea table. If i had sealed the hole i drilled it wouldn't have been as much of a problem, though that isn't a perfect solution.
A slightly better way to do it is summoner12's. Had i had access to a hole saw I would have done exactly what he did- glued the funnel to the bottom of the PVC and let it hang into/beneath the table. That doesn't address the problem of the screen runoff though. An "external' drip tray will.
You guys might want to check these out. I use them and they work really well under the cage. Just drill a hole in the center with a drain or funnel. Most of them are rectangular but they do have 24"x24" and 30"x30" square pans.