You could easily use paper mache to make the body and extra arms.
However, if you're inclined to being lazy or extremely cheap, you can get a really big trashbag, make leg holes, add a bunch of tissue paper, or any kind of paper to make it bulk out, paint some toilet paper rolls black and tape them onto the bag as extra arms, then just use some googly eyes from a crafts store as extra eyes. Maybe some yarn as your webbing.
I would think that material costs would be easily afforded in this case...What type of spider are you looking to clothe? I hear that spiders are self conscious about the size of their abdomens... so something concealing may be in order.
atm im using plastic bags that r red XD and for weebing whip cream that comes in those cans
lol...answer = totally unhelpful but funny as hell ;)
was not funny!!! coleens is hitting on u man
oh... sorry... I mistook the question!