About the same as Birddog6 (except the draft or angled sides of course). if there are hinges/catches you will need to start a pocket for them so the patchbox lays down flat on the stock.
I like to use a small dab of Superglue Gel to hold it in place while I am incising or cutting the outline into the wood with my pointed Exacto knife.
The reason I do this is I have found it is best to go around the outline several times to cut into the wood about 1/16 inch and to make sure it is fully cut.
A light whack with a plastic mallet will break the glue joint to remove the patchbox.
I have made some tiny little chisels and use these to chip out the wood from inside the profile cut, outward to intersect it.
This chiseling is about 1/32 to 1/16 inches wide.
The end result is the same as cutting a V as was described above.
Using the clean cut vertical edges as a guide, I then recut straight down to deepen the vertical cut. Then more chiseling to deepen the whole pocket but starting about 3/16 to 1/4 inch inboard to remove more wood.
You can remove the rest of the center area wood with a standard 1/4 inch chisel.
When the entire center area has been lowered, try the patchbox in the pocket. Using your thumb to hole it into the best fit with the pocket, use the pointed Exacto again to recut and deepen the patch boxes shape. Then repeat the operation of removing the wood out to the cut.
This may be required several times because of the taper you made on the sides of the patchbox but eventually you will end up with the patchbox flush with the stock and absolutly no gaps anywhere around the sides.
Have fun!
As a follow up to this here is a picture of the little chisels I made from some 3/32 dia music wire (available at Hobby Shops).
The picture doesn't show it but the faces are polished mirror bright and the cutting edges are razor sharp.
If you could stand the glare of looking at some of my rifle pictures, you can see that these have had a busy life in the inlay department. ::