What you Could use is called a pattern bit. It will have the bearing on the top of the bit to follow a pattern or other object you are cutting the opening to. Some will have pointed plunge type ends others flat ends depending on if you are through cutting or not. They are made for thinner/lighter cuts.
These work OK for a few light duty cutouts but for thick material in one pass I prefer to use a heavy plunge bit with a templete guide attached to the router. We plunge hands holds in 3/4 poplar this way. It takes a powerful router to do this. We use 4hp 240v units in the prodution shop.
There are plunge tip bits with the bearing on the bottom to follow a rear templete but they are hard to find and the bearing usually seases up from heat and debris real quickly.
