as far as using the box on it's side, i don't think that you can use it otherwise in a car. i'm using mine in-home and it sounds better laid on it's side, because the woofer was sag.
try diffrent firing positions. since it's got a square "footprint" you can try anything. i'm not sure, since i haven't tried mine in a car  ::). what sounds best is best.
i cut every round cut on my DBs with a jig-saw. i guess it can become very difficult to cut speaker holes using a circular blade    :-/.

The Jasper jigs available at http://www.partsexpress.com/webpage.cfm?&DID=7&WebPage_ID=3 are very popular on many forums. I bought mine locally at Woodcraft.

Those Rotozips do look sweet, especially next to my twentyish year old Dremel. As mentioned above, buying into the router plan gives other benefits. I have not seen a Rotozip do a half inch rabet for a flush mounted driver nor a roundover on edges, corners or port openings(large enough to consider them flared).

It looks like the 'zips have attachments for them to do everything but teach your plants to talk so a circle jig must be in the lineup. It may not have the ease or guarenteed repeatability of a Jasper jig but I'm sure it will be one more smile on your face when you see the results.