There are a couple of options. A shallow pot, with roots spread out widely can help, but you could also use a large normal, nursery pot with a dessert plate or tile under the roots to keep them flat and spread apart. Field growing is best, but not possible in cold winter climates for ficus. Another method, more advanced and tricky, is the ground layering (tourniquet) but indoors that would require a very large pot.

http://www.bonsai4me.com/AdvTech/ATfieldgrowing.htm

http://www.bonsai4me.com/AdvTech/ATNebari.html

Field growing would of course be nice, but yeah, out of the question for me. I think I may go ahead and use a large nursery pot with a plate as you suggested. With Gritty Mix it's going to be super heavy to move, but at least I don't have the worries about it being overpotted.

It is true with the few ficus I know, maybe all, that you can take a large, deep root mass and simply take a saw to it to make it as shallow as you need. If that's the case with your willow leaf, I would be using a deep pot for the next several years.

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while I would agree that this particular type of ficus, and most ficus in general can take quite severe root reduction, if it's possible to thicken the trunk and also develop a good root system at the same time, that would be ideal for me.