I'd get something with a hinge and a latch so it's easily accessible. Good point mentioned. It's best to have easy access to thsi shutoff. I think codes might even require it.
I wonder if you could use an electrical box cover without the outlet box like these: http://hubbell-wiring.com/Press/PDFS/H5189.pdf
Maybe go to a local jewler and have it engraved so it's labeled "GAS SHUTOFF" somewhere on the cover.
That got me thinking. .. I need to install one of those in my living room where our couch is so we can power the lamps on the end tables without an extension cord under a rug... which I know isn't the safest thing to do.