I use one small zucchini per box.
And while the water is boiling I use my food processor with the shredder attachment to grate the zucchini quickly.
Then I add the pasta to the water, and just before it is done (about 2 minutes before straining) I add the shredded veggies right on top of the pasta. Don't even mix them in, just let them sit on top.
I know boiling veggies is not the best way, but it's better than no veggies at all!
Since I loathe getting out my Cuisinart and cleaning it, I usually shred a bunch at once to keep in the fridge and use throughout the week, in baked breads, quesadillas, eggs and of course more mac n cheese!
Here are my kiddos sick as dogs today, whining about everything, but not about dinner. They love it!