All that a softbox is, is a piece of white nylon fabric held in front of the bulb, with a reflective-lined box surrounding the bulb and attaching to the fabric. You could easily make one out of a cardboard box, some white nylon, and aluminum foil. You could probably even make it fit over your existing light fixture. White foamboard is also a good material for softboxes:
http://thehowzone.com/how/Photo-Softbox/2
As for how big it needs to be, it depends on your subject. Ideally it needs to be as big as, or bigger than, your subject. So if you are lighting a human torso and face then it needs to be like 2'x3'. If you are just doing tabletop reviews of consumer products then probably 1'x2' would cover most things. Getting it as close as possible to the subject (to increase its relative size) is also important. You want it just outside of the camera's view.
Basically the idea is to create a large, diffuse light source, instead of a small, direct light source. This is the same principle behind lampshades. A large, diffuse source creates nice, even, "soft" light that is less likely to glare (although with a reflective surface like that Sony box, it would be hard to eliminate completely when held at the wrong angle).
I wouldn't shell out a bunch of money on a softbox; definitely play with a few home-made solutions first, because IMO you probably won't be able to get rid of the glare completely, but you can make it less likely to happen and less visually jarring when it does happen.