I just bought a parking sensor kit from Ebay for a relativly cheap price, spent a day (about 6 hours) taking my bumper off and installing the sensors etc, got to say, they work like a treat for a decent price!

Good bit of DIY this one and adds a bit of desirability and value to your car.

I've done something similar to mine, best tip I can offer, warm the bumper with a hair dryer just before you push the sensors through, you don't need the hole to be as big so you get a much tighter fit.

the kit was supplied with a 23.5mm saw which is the perfect size for the sensors so it waasnt required, different kits come with differebnt gear though