I've gotten ok results with 2 soft boxes one over the other as close as you can get them and still stick the lens between them.
It's not easy to shoot 'cause you're going to be standing with the bottom box pushing into your ribs and the stand for the upper box is what your're going to be pressing your camera against.
It's not perfect because once it's set up you really can't move around like you can with a ring flash, but you can get the ring like shadow and very flat light that is indicitive of a ring flash. If you have more time you can of course rig up something a little more flexible, but if you're going to be out of the studio till shoot day, this should work for you.
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