I've been using the RailChanger X with the 419 Shmedium bag for the past two years and haven't had any issues. Alternatively, you could try the GrayOPS plate with a Shmedium and a Game Changer bra. I've tested this setup a few times, but the bag didn't secure as well as I like compared to the RailChanger. Plus, the RailChanger makes it a lot quicker and easier to attach and detach the Shmedium.
It all depends on your shooting style. Some shooters, including myself, prefer to clamp the bag onto the plate, while others like to move the bag around. One advantage of not clamping the bag onto the rifle is that it allows for more flexibility to adjust, level, and balance your rifle on the go. Moving a rifle that's clamped to a heavy sandbag is definitely not easy—it's like handling a 20-ish pound rifle plus a 10-pound sandbag. For this, you might need a lighter fill bag. However, for a 10-round stage with 10 positions, clamping the bag onto the rifle can save significant time. I've often seen shooters fail to finish on time because they spend too much time moving their bag to next position during such stages. Otherwise, it's not a issue moving the bag on most stages.