If you’re not up for creating a full on Xenomorph costume, go smaller and make a Chestburster. Instructables user Fiery She-Beast made one from wood, foil, and clay. On top of that you’ll need an old t-shirt you’re okay with ruining and fake blood and a few other essentials. She’s how she made the little baby alien:
Stuff the pipe with tin foil, creating a lump on top of the pipe, this forms the alien’s head shape. Squash a few lumps of clay onto to pipe, and begin to sculpt the alien. It has a large forehead with no eyes, make the facial area smooth. The mouth has no apparent lips, but it has lots of sharp teeth, so be sure in include those. The temples in jaw consist of random lines and patterns with one main line running from the bottom of the jaw and curving upward on both sides. The tail is wrapped around the alien at the base of where is busted out. I found it fun to make a few intestines and flesh lumps around it from where it came out.
Read more at Instructables.