This simple box holds a stone to represent Earth, a broom for Air, a tealight candle symbolizing Fire, and a seashell for Water.
Why bother? Well, the obvious reason is that a portable altar is... well, portable. For some people, that's a desirable thing. You may wish to have a portable altar for any number of reasons. Perhaps your job requires you to travel a lot. Maybe you're a college student in a cramped dorm, and space is at a premium. Do you belong to a group that holds rituals in a different place each time? Got small children who will knock over anything and everything that you set out on a table top? Any of these -- and more -- are good reasons to creat a portable altar kit. It's easy to do, and it makes it a snap to just grab-and-go on your way out the door.
A small box can hold an altar cloth, and tools representative of the four elements for your rituals.
The first thing you'll need to do is decide what items you want to include in your portable altar. Some people like to put in every single magical tool they own, five different decks of Tarot cards, and their entire gemstone collection, but I've found that simple is usually better. In fact, if you keep just four items in there, you've probably got it made -- and those are the ones associated with the four classical elements.
Earth is symbolized by a pentacle, so if you can find a small one for your altar kit, add it. If you can't find one small enough to be portable, improvise. Use a small decorative dish, a small flat stone, or even a small vial of salt to represent earth.
Air can be represented in a number of ways, the traditional tool being the wand. If you don't have room for a wand, consider a feather, or even incense - the smoke is associated with both air and fire.
Fire is often connected to the athame, but if you're traveling around you may not be able to put anything with a blade in your bags. If that's the case, never fear -- use a candle (and bring matches or a lighter), or some other fire symbol. Deer antlers are also good substitutes for an athame. The cup or chalice represents water. You can carry actual water with you in a small vial, or use the cup as symbolic of water. If you don't have access to water, try carrying a seashell or some other symbol of the feminine.
If your tradition requires you to use other items, you can add those as well. Some things you might want to include in your altar kit are:
A small statue representing deity
Finally, add a piece of fabric to use as an altar cloth. It doesn't have to be big, just large enough to spread all of your tools on, so you can perform a working anywhere you may be.