I've never used a power tool to cut a slot in any of my boats (including a schooner with a 1/2" thick bottom that had 2 slots).

Instead, I traced out the opening with pencil and drilled a 1/4" pilot hole in each corner. Then I used a bonsai saw to cut from corner to corner, staying within the lines.

After that, I cleaned up and squared up the edges of the cut with a shinto rasp.

Both of these tools are razor sharp, so they cut through the okoume with little effort and reasonable speed. Since they are powered by you, it's easy to stop if there's a problem. They work at a human pace, so there's plenty of time to see if an error is starting and to correct it or stop and regroup.

They are also cheaper and much quieter than an electric router and its accessories. You don't need ear protection at all and can get by with reduced levels of eye protection and dust management compared to a power router.

Have fun,

Laszlo

Here's how I've done it on two boats.  Works perfectly and very little chance for something to go wrong.

Blog version

YouToube version

I have used both approaches, a router and Lazlo's saw/rasp method with good success.  I would submit that the best/easiest approch depends a bit on how the centerboard case fits the hull. 

For boats where the case extends through the hull, like Lazlo's Schooner or the CLC skeg kit, I would go with the saw/rasp.  You want the fit between the case and hull to be very tight.  You can use the saw to cut the hole a bit narrow then slowly and carefully use the rask to widen up the slot enough for the case to fit tightly.

On some designs like my skiff, the centerboard case does not extend through the hull.  On those, you generally glue the case in place first, then cut the slot afterwards.  In that scenario, it is very easy to open up the slot with a router using a flush trim bit.  First, you drill a pilot hole in the center of the slot.  (It is actually easier to get this right if you drill this hole from inside the boat BEFORE gluing the case in place.)  Next, insert router bit in the hole and move the router to one side or the other until the roller on the bit is resting against the inside of the case.  Then it is easy enough to move the router forward/aft using the inside of the case as a guide.  

One last point is that if you are painting the bottom, it is easy enough to hide any booboos with a bit of thickened epoxy and paint.

Thanks very much everybody.  While the router slot looks perfect, I think I'm going to go with the drill-saw-rasp approach until I get more comfortable with the router.  I appreciate the help!

   Hello,  The first time I ever used a router was on my Passagemaker last month.  It worked like a champ.  It was so simple and wonder why I was nerveous.  There is a first time for everything... go for it!    Best, Mike