The grills are acoustic cloth which you can pick up very easily from car audio shops and fit over a simple wood frame. I've used it before back when I had a car and fitted a stelth parcel shelf.
Wilmslow Audio (
http://www.wilmslow-audio.co.uk/ ) are a really nice bunch here in the UK that might have some bits to help you build something to fit, including the little rubber bungs that mount into the holes on the front baffle. I'd suggest you drop them a mail and see what they can do for you.
Personally I'd just run them without. Very few speakers sound better with the grill in place.