I was going to a Halloween party in two days and wanted to make a Homemade Marvin the Martian Costume. So I headed over to Walmart and picked up a whole bunch of foam, and found that they had duct tape in green (didn't know it came in that color until that day) and some black fabric I could see through.
I took all this home and went to find something I could use to shape the helmet out of so I looked around and tried a big pot but it wasnt big enough, so then I noticed I had a garbage can in my room with one of those dome swivel tops on it (I actually used a smaller one of these for a bender costume I made ironically) but it turned out to be the perfect size so I took some foam and started forming the shell of Marvin's helmet using the green duct tape to hold it together. Eventually it started taking the right form and I was able to remove the trash can lid.
I then took some more foam and wrapped it around my head and taped it so I could easily remove it. I ripped holes for my mouth and eyes so I could see, then covered the piece around my head with two layers of black see through fabric. I pinned the fabric on and later super glued it so it would stay on. Then the moment of truth came when I put the green helmet over the black face foam part to see if it was proportional. It was and I was filled with joy. I then cut a couple pieces of computer paper for eyes and put them in roughly the right locations which happened to look ok for other people and didn't actually block my sight at all.
Seeing as I ran out of time, I didn't make the trademark antenna on his hat or wear the shoes, or the belt. I just wore some black jeans for his spandex skinny legs and a red turtleneck for the top.