For strong attraction to metal you want both poles of the electromagnet to touch the metal. Thus it would help if you welded another piece of (magnetic) material to make a triangle or U-shape magnet.
Don't know what best to power your magnet with. It would likely require many tens of amps at a few volts to generate a good magnetic field which is not easy to generate from standard power sources. Winding the coil with many turns of much smaller wire makes its much easier to power. It's the ampere-turns that determines the field intensity. Thus 20 turns at 1A is the same as 1 turn at 20A. Look at a wire table to determine the wire resistance for various size wire, and from that you can calculate how many feet of wire for a desired resistance for a given current and voltage.
For example, 28AWG wire has a resistance of .0649 ohms per foot. Thus if you wanted to power your magnet with a single D-cell at 1A of current (1.5Ω), you would wind 1.5Ω/.0649 = 23 ft. of wire on your magnet. If you used two D-cells, you would wind 46 ft. of wire, for about double the ampere-turns.