FlxTileblock is a quick and simple way to make rectangular, tiled objects. It randomly places tiles out of the assigned tilesheet. You can also specify to include ‘empty’ tiles, which will leave more gaps in the finished block.

tileblock.loadMap('path/to/tiles.png', 16, 16, 1);

Demonstration

package;

import flixel.FlxG;
import flixel.FlxState;
import flixel.tile.FlxTileblock;

class PlayState extends FlxState
{
	override function create()
	{
		super.create();
		
		var block = new FlxTileblock(16, 16, FlxG.width - 32, FlxG.height - 32);
		block.loadTiles("assets/tiles.png", 16, 16);
		add(block);
	}
}