Not all Santa Muerte followers have the luxury of being able to erect a permanent altar for our most Holy Saint Death, but that doesn't mean we can't work with the Santa Muerte and make offerings to her. With the help of a temporary or travel sized altar, we can work with the Santa Muerte just as effectively as anyone who constructs a permanent altar. As with a permanent altar, the most important ingredient for building a temporary or travel sized altar is an image of the Santa Muerte.
One option is to take a small box of any size (cigar box, wooden keepsake chest, Altoid can) and paste into the inside of the cover an image of the Santa Muerte. You can print an image at home or purchase a small Santa Muerte prayer card. If you're able to find a tiny figurine or medallion with her image, you can also use a hot glue gun to attach this image to the inside cover. In the rest of the box, store appropriate sizes of things that you can offer to Santa Muerte when you have the chance to open your tiny altar, like candles, cones of incense, vials of perfume, etc. For just about any offering you would make to the Santa Muerte at a full sized altar, chances are good that you'll find travel sizes of those same offerings. Cigarettes, tiny bottles of liquor, fun sized candy bars, pieces of bread, small vials of perfume, etc. all make excellent options for a small altar box. You can keep perishable offerings in your box until they begin to go bad, spray perfumes in and around the box, or even pass the box through the smoke of incense, a cigarette, or cigar. What matters most is not the mechanics of how you make your offering, but your intent - always remember this.
As with a full sized altar, your temporary or travel altar can rest atop a piece of fabric or cloth of your choice. You could use a square of fabric or cloth under your altar box while open, and to wrap your tiny altar when not in use.
Another option is to simply carry a medallion or prayer card of the Santa Muerte, and to carry it in your hands when you want to communicate with Saint Death. Offerings can be made by sprinkling perfume or liquor on the image, passing the image through the smoke of a burning offering, et cetera. You can carry the image around your neck or in a small satchel that can double as your altar cloth if you want to place the image on a flat surface - as you may want to do to free your hands for other ritual work.