I made a simple UFO out of (what else?!) "saucer" sleds!
First, Tape two saucer sleds together and cover 'em with aluminum foil.
Then tape a big bowl on top (for the 'cockpit') and cover with foil, too.

Next, I used an old rusted bird cage bottom (that someone was about to trash)
and spray painted it with some Rust-Oleum Metallic spray.

Use some black felt to make some windows and you're done!

Here is it hovering over a bush with an Alien behind it.
I attached an alien to my birdhouse pole with some twine.