I would fill a milk jug (with a screw-on cap) or empty soda bottles with water & use them as ballast if you don't think you'll need the soil volume. That, or turn a clay or plastic pot bottom up and set it on the main pot's bottom. Cover the drain hole (in the over-turned pot) and fill around and above the filler pot with soil.

Al

To be honest, those containers aren't that big, but I guess that's because I have plenty that are bigger, for growing veggies. I'll tell you this though, should you use the 5-1-1 mix, it won't cost you much to fill the whole thing. Otherwise, Al has some good ideas. As for what to put underneath, either buy one of those plastic trays, available pretty much anywhere, or use some egg crate material. Even a couple of pieces of pressure treated 2x2's cut to size, will get that pot up off of your deck, so air can dry out the drainage water.

Joe

packing foam peanuts.

"I'll tell you this though, should you use the 5-1-1 mix, it won't cost you much to fill the whole thing"

I'm not familiar with 5-1-1 mix?

It's a custom made mix of pine bark fines, perlite, & peat moss. Rather than going in depth, just use the search box on gardenweb, and type in "5-1-1 mix". You'll get tons of info on it. I use it, and this year, for my veggies in containers, it costed me a total of just under $20. For pots, I have 2 huge 24" pots, 2 planter boxes that are 3' long, 1' wide, and 1' deep, and I also have many smaller pots that I filled up for annuals, and a few houseplants.

Joe

Don't use foam pellets or anything that you have to pick out when you dispose of the potting mix in some year in the future.