As Stephan Matthiesen notes in the comments, if your pizza box is mounted on a standard bicycle rack, there are rear lights that attach to the rack and should be visible even with the box on the rack. Or you could do as Glenn Stevens suggests in his answer, and get a light that attaches to the seat stays of your bike.
If you really want to attach a light to the pizza box itself (and don't just want to glue or bolt it in place — I assume the pizza place might complain about that), you might want to consider a red clip-on light, or perhaps a little magnetic light like these.* These are meant to attach to clothing or to saddle bags, but there's a good chance that either or both of those might also attach to the clasp on the pizza box, if it's metallic and/or has some kind of a handle you could clip the light to.
Or, if you like the idea of a magnetic light, but the ones sold for joggers feel too small, you could probably make one yourself by supergluing a couple of neodymium magnets to a basic cheap tail light (possibly after hacksawing any extra bits out of the way).
Those products also suggest yet another alternative solution: instead of attaching the light to either the box or the bike, attach it to yourself. Besides the clothing-mounted options mentioned above, there are also rear lights that attach to your helmet.* I assume your pizza box isn't so big that it would obscure that.
Also, remember that for safety, reflectors are just as important as lights, if not more so. You should be able to find some reflective safety tape or stickers that you can attach to either your bike or (if the pizza shop permits it) to the pizza box. Also consider wearing high-visibility clothing, or a reflective vest over your normal clothes. Wheel and/or pedal reflectors are also highly advisable for urban night riding — they might look "dorky" to some, but their distinctive motion instantly screams "bicycle!" to any driver whose headlights you may find yourself in.
*) The linked products are random search results from Amazon for illustrative purposes. You local bike store, or even a well stocked supermarket, will likely have something similar.