I found a spider prop at Walmart last year similar to this.
Amazon.com: Spider 23In Hairy Poseable Prop:... . I just hot glued a Velcro strip to the bottom of the spider and sewed another strip of Velcro to a black hoodie. I was trying to improve on the costume for this year and got a black baby onesie to use, but I'm not sure if this is an improvement. I cut black stretchy knee high's to use as his front legs and connected them together at the back of his neck.
I just found this on Amazon
313g2LP7jfL__SL500_AA280_.jpgand it would make a cute costume!