Hi kbec,

I have exactly just that - a 4 x 8 raised bed that is one foot tall sitting on my driveway. Then I have another 2 x 16 raised bed that is also a foot tall sitting on the driveway. I've got pictures in my blog of this.

Now - making it movable - that is something that I can't provide assistance on. You are looking at a huge amount of weight. You'd need some heavy casters with locks (so it didn't just roll) and some very heavy board underneath to hold the soil up. Even then, the boards underneath would most likely decay if using something like plywood or particle board.

You could use slats of treated lumber as the bottom - and this would also allow for drainage. But you'd also need to run some supports across the bottom so the boards don't bow in the middle. As you said though - the soil is going to be very heavy - and I just would wonder how easy it would be move something that large.

Here is a link that might be useful: BsnTech Gardening Blog