Jasper Circle Jigs
Sure, you can make your own trammel to cut holes with your router, but why bother? Jasper Audio (Fellow Houstonians!) made a great jig already. I own the Model 200 and the Model 400. If you only get one, get the Model 200. The jigs are calibrated to be used with a 1/4" straight bit, but any size will do. You'll just have to adjust your cut accordingly.
Only use the Jasper Jig with a plunge router.
The Model 200 will cut circles in 1/16" increments from 2-1/4" to 18-3/16" in diameter.
Model 200 
The Model 400 is perfect for tweeters and small midranges, cutting holes from 1" to 7-1/2".
Model 400 
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Jim
Houston, TX