They're awesome, affordable, and solid - HERA Arms AR15 Side Folding Unit Adaptor

I now own 6 of these. My last one had a teeny bit of play in it, but in that situation you can cut a little square of electric tape and place it at the bottom/back of the inner pocket where the two main flats close against each other. If there's up/down play, place tape on the flats at top and bottom. Can't even tell it's there and it'll tighten it right up. Add more layers if needed but you shouldn't need more than one, if you need anything at all. Easy to install, the pins lock it in place, but I still vibratite mine to make sure nothing loosens up.