The only really tricky bit is the cut for the tweeters because the drivers are inset (and therefore don't overlap the veneer - and cover up inaccurate cutting).
Does anybody have any advice as to how I can get the cuts for the tweeters done nice and cleanly?
The flush bit will only get you halfway there with inset drivers.
Here is one method to get a perfect cut:
1.) Use the flush bit to trim the veneer the size of the tweeter through hole.
2.) Use a band saw or adjustable hole cutter to make a disk the exact size of the tweeter hole out of scrap.
3.) Use hot melt glue to attach the disk in the tweeter hole. Don't use too much glue, just enough to hold it in place. (3-4 dots)
4.) Make an extended router base with a pivot hole.
5.) Screw the router base to the center of the tweeter disk and route the recess for the driver.
You may have to reverse steps 3 & 5 because the attachment screw will probably be under the router base.
Or -
If the veneer is real thin you can probably sand it with a curved sanding block to match the recess once the hole is cut.
Wayne