http://www.drill-bushings.com/
I've made a guide on the cheap once. However it was not as critical and the hole was unmolested. I took two long bolts app. 2 inches longer from the surface being drilled. And a piece of good card board. Transfered the pattern on the card board, slide it on the bolts 2 inches away. The whole point was to give a reference to steady my hand and drill.
Ralphy