Here is my husband's costume. (Click on the pics to make them larger)
He named it Speedy and held a stick with a carrot (to encourage the snail to go). He found a picture of a similar one on the web for inspiration. There were no plans, just trial and error. It is made of chicken wire, batting, fabric, and pool noodles. The eyes are Styrofoam balls. It has straps to hang off his shoulders and he wore it several times for various events. It took about a month to make (work nights after work). I had a slight less elaborate costume as a female yard gnome to compliment his.