If you have access to a drill press then an appropriately sized hole in a suitably thick bit of square wood can work a treat. I used this technique for drilling the dog holes on my bench and each one came out square and true:
If you're going to buy one, don't get one made of plastic. It really needs to have some kind of metal bushing else it won't last five minutes. I think knocking one up with the drill press is you're best bet really.
I use one of these sometimes. It does the job. Although it would be better if it was transparent and raised off the surface a little so you can see what you're doing.
