Hi Bubba,
I've used very large flower pots for just that purpose. I was barely able to carry them back inside for the winter but it did work quite well. The thing is to use bonsai soil because you also want good ramification of roots and that can run into $$$ (don't tell anybody, but I re-use my bonsai soil after sifting out the broken-down organic material). I used some plastic dishpans (the rectangular kind from any discount store) and punched lots of holes in them. I also used big round plastic pots intended for hanging plants because they are fairly shallow in relation to their width. If the copntainers you decide upon are deep, you might want to set the tree on a slab of some kind so that the roots fan out instead of going straight down.
You'll need to do this for several years if you want to get really good results. I got a bit impatient with mine and those tubs were taking up a lot of room in my townhouse so I quit after the second year. The growth was noticeable but not spectacular.
I hope this helps.
Krys

Here's another option and something to think about. Jumping a plant from a coffe cup sized pot up to wash tub or larger gives it lots of room, and also lots of unused soil that can be a place for unused, undrained water to sit an cause problems. There is some thought now that "overpotting" may not always be as benificial as it seems.
One strategy you may use with your ficus is to take it out of its small pot and put it into a larger but not massive pot-a plastic dishpan would be great. Use good bonsai soil, work the roots to achieve good spread and then bury up to the rim of the dish pan out doors in an appropriate spot. You willow leaf out to grow like the weed it can be and come fall you will have a good sized plant. You will want to cut the branches back some what so it is more managable indoors over the winter. COme spring start the cycle over again- trim back the winter growth, pop it out of the dish pan and hard prune the roots too ( it is a ficus you can take a lot of root off if the tree is healthy) Put it back into the dishpan and do it all over again. This gives you the big trunk you are after with out having to deal with monster size pots and long masses of tangled roots. If you prune roots and branches with some though you woul dbe able to put this into a real bonsai pot in a year or two, depending on how large a trunk you want.

Thanks for the advice!

I understand using a plastic dishpan and punching holes in it. But, if it was then buried in the ground, would the water run out? Im just imagining the drainage holes getting clogged and root rot to follow. Or would the earth actually pull water out of the pan like a sponge?

Rot rot is much less likely when the pot is planted in the ground for exactly the rason you thought of-the soil around the pot will tend to pull water out of the pot itself. Being buried in the ground also moderates temperature and humidity changes.

Okay here is a question about open ground planting. I have a small area that my wife has alloted me and promised not to put any flower beds. My problem is that the soil in my area could be used for making my own bonsai pots....PURE CLAY! I put a procumbens in the area 2 years ago and while it has increased the trunk size dramatically, the plant still wobbles. I dug a good sized hole and mixed in a bit of the clay with some good bonsai soil. I think the roots have hit the clay and it acts like a brick wall that they cannot penetrate. So do I need to till the whole area and add something to loosen it up or will it work fine as long as I put the potensai into decent bonsai soil?