She generally looks rather bedraggled after crawling through ventilation ducts and running down corridors, and the costume part for her, combat trousers and a white vest, was very basic and straightforward (in theory anyway-I spent a horribly long time looking in charity shops for them- a bizarre lack of these items available!).
For the alien part, I decided to just make the head, out of papermache. In case anyone feels an urge to make a xenomorph head mask thing, here is how I did it. It took about 1.5 hours excluding drying time.
First I made a tube out thin card for my head, which wil act as the alien's neck. I marked roughly where my eyes and mouth would be and cut them out. I made the tube about 20cm higher than the top of my head.
I then started on the main head bit. The alien's head is curved and rounded at the ends, so to create the base of the curve I used plastic bottles. I used the large 2 litre ones, cut both ends off and slit down one side. I opened them up and made card pieces to hold them open.
I then cut three large rectangles of card, and stapled them together on a slight angle so that they make a curve, and then attached the bottles onto each end.
I put two long strips across to hold the thing in shape, and two across each end under the bottles.
To make the mouth part, I cut a lower jaw out of a folded bit of card and added a bit at the front to complete the jaw at the front. I then cut out the top jaw and stapled it on. I then attached the jaws to the bottom with masking and duct tape.
I then attached the neck to the top with duct tape, and covered it with papermache.
I then slightly ran out of time as I was going to have tea with my parents, so my lovely flatmate charlotte had to paint it for me. Luckily it just fit in the airing cupboard so it didn't take long to dry. She used a mix of poster paint and PVA so that it had a slight shine. I tend to use water based paints for costumes as they wash out of everything, though that turned out to be a bit of a drawback at the party as when I spilt my beer on it by accident the paint came off all over me and other party goers!
So here are the final costume options,