Thread: Routing an inside hole at 45 degrees???

I'm trying to get started on building a headboard/bed for my new place and I'm currently at a impass.

The plan is to make this thing look as if it were one big slab of wood 8'x8'. It will have 6 "windows" in 2 rows of 3 that are open to the wall behind. The only way I can figure to achieve the look and hide the joint is to cut the holes out and 45 the edges. I then plan on building a box with 45'ed edges to join to the hole. I basically want this to look like a square bored hole.

The problem is what bit to use and how. I can't think of any other way to get an accurate 45 on the inside hole other than a hand held router. The only bit I can find that seems like it'll work is a lock mitre. But they are all saying for use with a router table.

Any ideas? Could I use the bit with a hand held? How could I set this up to ensure an accurate cut? Since the hole would already be cut out, how will I ever keep the router level?