It's a bitch. The method I used was to get take the radius of the circle that is the "rounded" part of the corner and cut a circle of the appropriate diameter (allowing for cutter width) out of a square made of something stiff enough to make a router template. Before cutting the circle, mark lines on the square between the centre points of each side. Then carefully cut along the lines to obtain a quarter segment of the circle. Repeat (because you need to avoid using the pieces which were on the side of the line that the cut was made).
You now have four 90 degree sections of the right diameter for the rounded corners. Cut out a square to match the dimensions of the square sides of the speaker. Glue and (countersunk) screw the segments of the circle to the inside corners of the square. Half a day later, lots of swearing, and you should be there with a template. Repeat times your number of drivers.
Oh, someone must have a better method, that doesn't require one of those "tracing" router fittings that cost a fortune.
First, you'll need a top bearing pattern bit. I use a 3/4" straight bit with a 1/2" shank. Buy a couple 3/4" bearings to fit onto the 1/2" shaft.
Cut ~12" square(if that's big enough for the driver) of 1/2" MDF. Route a circle into the center of this piece the same diameter of the drivers rounded corners. Cut some strips of 1/2" MDF about 1 1/2" wide. With the driver face down, cut and position these strips around the square portions of the driver, butting them into each other. Hot glue these together. Take the square you just made and position it over the round template so that the driver fits thru both of them. Hot glue the templates together. Flip the template over, onto another piece of MDF(that will be the final template) and screw it down. Set the router so the stacked bearings will ride on the both layers of MDF. Route the final template.
It's easier and quicker than it sounds! Pretty foolproof as well.
Ive found that a cheap collar setup works nicely if used carefully. They arent as accurate as using a template/bearing guide but good enough for the task. I make my own but a commercial one like Vermont will do. First, cut the hole for the speaker. Next, put the speaker in the hole and line it up in the finished position. Mark the screws holes w/ a pencil. Be accurate. Now lay the speaker face down on some template material. Use good ply or something that wont splinter. Now make an outline of the speaker. Using a divider or compass, set the distance between the edge of the bit and the outside of the collar and then follow the speaker outline all the way around, being carefull to keep the divider perpendicular to the outline. Mark the screw holes also. Take a straight edge and make lines through the screw hole marks from one corner to another and side to side. This is so you can line up the template after you cut out the center (removing the hole marks)Make the same lines on the enclosure. Cut out the template. You now have a square hole exactly the collar width larger than the speaker.You may want it slightly larger so as to avoid too tight a fit. Mount the template to the enclosure(screws, carpet tape, clamps etc) set the depth of hole and slowly remove material until the face and sides of the cut are smooth. If you have to make a deep cut, dont try it in one pass. Set the depth to half and make two passes. Its a hassle but if you chew up the enclosure or burn up a bit youll be in for even more work.
This method is simple and requires few accessories. The quality of the finished product is reflected in the care you take in the layout and the template fabrication.
Pete Mazz's suggestion is a good 'un, if the corners of the driver are actually equivalent to a circle whose centre is in the centre of the driver. Mine weren't. They had smaller radii than that, hence having to chop the circle up and paste the segments onto the corners seperately. Other than that, pertty much the same technique. Check the driver drawings carefully - mine were Focal too!
If your baffle depth in the box is already set or if your boxes are assembled, then you have to go with Jake's of Pete's methods.
I've done a couple boxes for JBL LE 10A's and the first time I did as above. But the last time I got a piece of 3/16", void free birch plywood from a marine supply. Cut the plywood just a bit larger then your baffle. Place your speaker up-side-down on the plywood where it is to be mounted and trace the speaker frame on the plywood. Carefully cut it out with a sabre saw and file to the lines. Then you glue the plywood to the baffle.
Birch has a very nice grain pattern so you can finish it however you like and it turns out looking great. You also form a composite baffle with two different damping characteristics.