I have a Yamaha P115 keyboard. When using the double X brace my keyboard wants to slide off as there is no stops built into the brace. How can I stop the movement before the keyboard hits the ground?
Make sure the top tubes of the brace (the ones you place your keyboard on) are level.
If the brace is level and the keyboard still slides around, you need some high-friction padding (e.g. rubber or foam) between the tube and keyboard.
Rubber stops are fairly common on these stands:
But they're easily lost: they aren't locked in place so they'll slide off when you apply too much force.
Use velcro tape - hook and eyes - glued on the top bars of the stand, and the matching places under the keyboard. And if the stand is to be taken out of the house, pop rivet or use adhesive on the endstops. Otherwise they will eventually escape.
I guess you mean the keyboard slides towards or away fom you. that's what the endstops are there for. If you mean it slides sideways, then a couple of strips of wood, or carpet, or rubber glued under the keyboard will stop that.
I have used foam for lagging water pipes split and slipped over the top tubes, too.