Trash bags, shopping bag, and tape.
They are light weight, take up little space and will keep you mostly dry.

Rip a couple holes in the bottom of one bag to stick your legs through,
rip some holes in the bottom of the second bag for your arms and head,
tape up the seams, stick each foot into a shopping bag, tie up the top,
and tuck it into your socks.

It works well, but looks funny if appearance is important to you.