Of course you can use a router as some have suggested, but unless your flush mounting the drivers where you need to make a rebate, then a jigsaw will work fine. Mark the hole out with a compass and pencil, and cut it out with a jigsaw - since the driver mounts from the front, you cant see if the hole is slightly imperfect and it wont affect anything if it is, you'll be cutting right on the line anyway. Jigsaws in Australia cost as little as AU$20, so I'd guess thats about US$17. You dont need a fancy one, the cheapest one available will work.