I would warn against the Wolfcraft guide. I have plenty of experience with Wolfcraft from long ago, and there is one thing you have got to keep in mind with their products: if it moves, then very soon it will move too much.
The Wolfcraft guide swivels, so you can drill under different angles, and you can bet that very soon it will not ever be 90 degrees anymore. Waste of money.
What I like about the one from Axminster that Peter shows is that is fixed so you're always sure it is at 90 degrees. You can always order it directly from Axminster, they ship to the States.