The domino releasing mechanism is a little bit tricky. I made my domino blocks to be just slightly less than one Lego stud (8 mm) thick and slightly less than three studs (24 mm) wide. To push the dominoes out reliably I needed a stop for the dominoes that was not exactly on a stud boundary. In retrospect, if I had made my dominoes just over one stud thick it would have been simpler. I could have had an opening on the side that was two studs wide, and push out one domino with a slider that was one stud wide. With 9 mm thick dominoes, a two stud opening of 16 mm would prevent a second one from being slid out, and a one stud thick slider would never catch a second domino by accident.
But I hadn't thought this part fully through when I made my blocks. And besides, the thinner blocks are more elegant.
What I ended up doing is to make an opening that was effectively one and a half studs wide by restricting the front of the opening where the slider is by half a stud with some Lego pieces mounted at an odd angle. You can see the bit that sticks out in a funny way on the bottom and the side-mounted yellow piece on the red 1x2 brick with a hole in it.