how do I use bounding box

[IvanoVladimirs]

for example I wanted to place a bounding box under me. How'd I do that?

[END3R1355]

Code from @Du-Couscous matrix fly

if (mc.theWorld.isAirBlock(new BlockPos(mc.thePlayer.posX,mc.thePlayer.posY-1,mc.thePlayer.posZ)) || mc.thePlayer.inWater) {
            if (mc.thePlayer.fallDistance >= 1) {
                mc.theWorld.setBlockState(new BlockPos(mc.thePlayer.posX, mc.thePlayer.posY-1, mc.thePlayer.posZ), Blocks.barrier.getDefaultState());
                blocks.push(new BlockPos(mc.thePlayer.posX,mc.thePlayer.posY-1,mc.thePlayer.posZ));

[IvanoVladimirs]

@end3r1355 thing is i just want to set a colliding bounding box under the player