I got this idea from the family handy man magazine. This is a very simple jig for ensuring you have 90 degree corners when building cabinets, boxes etc...
You don't have to make the whole in the centre. Below is a snapshot from the family handy man magazine and their version was quite simple.
The main thing to ensure is that the jig is square and you have the ability to secure with clamps.
Here is another action shot :)