You can quickly identify the blowfish, also known as the puffer fish or the fugu in Japan, by its spiny, round shape and puckered mouth. For a fish buff, the puffer fish is a natural costume choice since the cute, puffy-looking fish is actually highly poisonous. In fact, according to the book "Field Guide to Seafood" by Aliza Green, the common blowfish carries around enough poison to take out 30 adults. Recreate this innocuous looking but poisonous fish using inexpensive craft store materials.
Turn the T-shirt inside out, and place it on your work space with the bottom hem facing you. Measure around your legs directly under your butt, and subtract about 15 cm / 6 inches from that measurement. Cut your elastic to this length.
Hand-sew your elastic ends together to create a loop, or just pin them together with a couple of safety pins. Fit this elastic band over the bottom hem of your T-shirt. This should create a gathered effect.
Fold your shirt's bottom hem up and over the elastic band. This creates a gathered bottom for your T-shirt. Close the new hem you created by fastening it with safety pins every 2.5 cm - 3.8 cm / 1 to 1 ½ inches. Turn the shirt right side out.
Tear your aluminium foil into 15cm x 15 cm / 6" x 6" squares. Roll these sections into medium-sized balls and then grasp and twist the tops of the balls to turn them into a spiny cone shape. Spread and flatten the cones' bottoms. Make enough cones to cover the shirt to your satisfaction. Start with about 50 cones, and make more if the blowfish costume looks sparse after you've attached all the cones.
Insert the cardboard square into your T-shirt. Heat up your hot glue gun and glue a dab of hot glue to the bottom of your foil cone. Attach the glued end of the aluminium cone to the shirt. The cardboard insert should keep you from accidentally gluing the front of the shirt to the back. Continue attaching spines to the shirt until it's covered, and remember to pay equal attention to the sides and back of the shirt.
Cut your foam ball in half with your knife. Paint each half black. These will be your blowfish eyes. Glue their flat sides to the headband using craft glue; hot glue might melt the foam. Glue one to the top left side and the other to the top right side of the headband to mimic the spacing of fish eyes.
Put on your tights and long sleeved undershirt. Put on the T-shirt over it, and arrange the elastic so it sits tightly against your body. Stuff plastic bags or other stuffing material into the shirt through the shirt's neck. This will "puff up" the shirt, and the gathered elastic bottom hem should easily keep the bags from coming out of the bottom. Pull the elastic headband onto your head so the top of the band with the eyes you made rests against your forehead. Draw a large, red, puckered mouth over your own lips to complete the costume.
If you want a longer lasting blowfish costume (one that needs to last more than just an evening or two), hand or machine sew your costume instead of using safety pins.