If you're trying to make this a stand-alone kit (that is, not dependent on your web gear or pocket itmes), that gives a place to start.
Obviously water is a major concern, so in addition to whatever you're carrying on your webbing I'd add one of the .5 quart pilot flasks, the kind that's designed to fit in the thigh pocket of a flight suit, and keep it full. In case you run across a water source I'd add at least one or two Platypus bags or similar "foldable canteen". Also, some form of water purification tabs. To boil suspect water and for general puprose I'd go wiht the largest Sierra cup that you can fit in your pouch.
Food should be calorie dense and no-heat. Stuff like Datrex lifeboat rations comes to mind, as does hard candies and glucose tablets for quick energy.
Fire can be covered any number of ways, but my favorite ferro rod is the Gerber Strike Force. It's a big rod, throws mondo sparks and you cans tore tinder in the handle.
Shelter is problematic in such a small space, given the area. While a large garbage bag (the 55 gallon contractor size) is handy, I don't know how it wuold work in the desert. A loosely woven, sand colored piece of cloth would be better, but take up more space/weight. For sure take lots of cordage and light wire for lashing together a shelter. Also a sewing kit to repair your clothes.
Knife-wise I'd carry a Ranger SAK in your trouser pocket (that's the model I carry and it has a good assortment of tools for improvising in the field), with some sort of multi-tool on your belt in a pouch. If you want a knife specifically for your pouch I'd go with a large "tactical" folder such as the Manix or the SERE 2000. If you know a friendly quartermaster maybe he'll order you up a Benchmade 9050 Auto; that's my personal survival folder of late, and it's in inventory. For a belt knife (on your trouser belt) you should go wiht something NOT military, with a 3 to 5 inch blade and comfy handle that's relatively stout. I'd suggest the Grohmann #1 Camper, flat ground, or if you want to save some money a Mora or a puukko. A Mora Clipper comes wiht a decent sheath and is a good knife and will only set you back about 10 or 12 bucks; the Grohmann will be closer to $40.00.
Add a small first aid kit, with the basics covered plus whatever meds you might need, and maybe a couple of combat dressings besides (also handy for firestarting) and you should have most of the bases covered. Given the area, I'd reccomend adding a canteen, carrier and cup to your list of "always on you" items, so thjat you have more options for cooking and carrying water than just your pouch kit.