CMMG released this product at SHOT (yet somehow forgot to mention it to me when I stopped by their booth), but they’re here and showing off some of their production models at NRA this year. Their aluminium upper receivers are produced with two open rectangular areas on either side, into which screws either a flat closed panel or an ejection port. It allows the shooter to buy a single gun and easily swap it from right hand to left hand.
CMMG has just now reached full production on these things, so keep an eye out.
The thing I keep wondering is who would actually buy this thing? I’ve never heard of anyone suddenly and without warning switching from a righty to a southpaw, so I’d imagine that one style of upper would be sufficient for your needs. But I guess if you want to allow yourself the option down the road of cheaply swapping to a slick-side AR this would be the way to go.