You'll need to decide whether you want to use a box or a bag. If you plan to carry your altar kit on a plane, backpack, purse or other place where space is at a premium, go with a bag. If it's something you're going to keep in your home, or maybe take over to a friend's, you can probably use a box.
The great thing about a bag that enhances portability is that your bag can double as an altar cloth. To make a circular piece of material into a carrying bag, simply stitch a 1/2 hem around the edge of the circle, and run a cord through it. Knot the cord at the ends, and then when you pull it tight you'll have a drawstring bag that unfolds into a round altar cloth.
If you want to use a box, great -- there are tons to choose from. You can get a plain wooden one from a craft store and paint it or decorate it. You could use an old cigar box and cover it with fabric and embellishments, or you can buy a pre-made carved or decorated box from one of the thousands of retailers who specialize in metaphysical gifts.
The key here is to pick a container that will hold the tools you need.