Looking for a custom halloween costume...

get yourself sprinkler tubing for the cap and the stem bottom, a single color drawstring skirt from the plus size part of Goodwill/ thrift and a similar color shower curtain.
and paint lots of paint.
sew/ staple the shower curtain into a tube you go into, and pin it over the shoulders
you might want a ring of "hula hoop" tubing (kiddo size 3/4 or even 1/2 inch) to weight down the bottom.
The skirt is the cap: tie the waist together and make sure the drawstring is inside
glue the hems over more hoop tubing.
Use a small hoop on your heead so the cap stays put and paint away.

Now, if you had a couple grand we could make you this: (this is my hubby's work)

 

yep, atom bomb. I was thinking of a no fabric cutting cheapie, but yep: sheets and two different sized hoops. bigger is the cap. a full freaking roll of the tubing stuff is around $15 max. And you have hoop materials to use/ swap/ gift.
for paint, price shop the craft stores, and look for art stores. I actually pay far less at an art supply store than Michaels/ Hobby Lobby (and can you ever find it at jo anns?)
any latex paint will do. ask the hardware store if they have drips and drabs of off mixes, they often sell that cheap.

 

BTW, I ve seen this as a jellyfish, too. add holiday garland and feather boas for the tentacles (and you can say it is inspired by poster artist Michael Everett's 1999-2000 NYE run costume with Cheese.)