Best? Your choice! But the choices are:
1. Build it permanenly heavy. Ouch, my back!
2. Load it with bricks after it's in the water. Tedious.
3. Free flood ballast. Works, but still heavy when you try to pick it up.
4. Pumped system. How many ways to skin a cat? Here's one way to pump ballast:
http://www.greatlakesmodeling.com/ti..._tank_pump.htm

Pat M