After doing a spot of shopping, I worked on making a side wall for my customized plow. I could buy one (or two, I'm not sure if I want one one each end) from Warn for about $80 each, but I wouldn't be a member of a site like this if I did something crazy like that.
Here's pics of what I've done so far:
It's roughly based off the Warn 80607 side wall (online images of the unit, as well as the install instructions), but mine extends out more and it also is bolted to the face of the plow, not just the end of it.
Tomorrow (well, later today) I plan on boxing in the support braces, bolting on a strip of a conveyor belt to the bottom edge to close off that edge while still being able to go over uneven pavement without catching the sidewall on it, and round off the corners a bit so they aren't so ouchy.
I'll try it out a bit tomorrow, but there isn't any unplowed pavement around (well, that I know of) that I can do a proper test of how well it works.
Once I'm able to do a real test of it, I'll decide if I should make another one for the other end...