you might have seen instructables just like this one before, but they don't seem all that sturdy.
I have made a knex flail that is relatively strong, and easy to make
Step 1: What You'll Need
(the chain can be found in the screamin' serpent or corkscrew canyon sets)
Not pictured: knife, paper, duct tape (optional)
Step 2: The Handle
Step 3: WAIT!
(use the pictures for help)
Step 4: Chain
you can make it as short or as long as you want
Step 5: Ball
cut a hole in it with the knife, and stick a yellow rod with a semi circle into it and tape it on
Step 6: Connecting Ball to Chain
add it to the semi circle on the ball and connect the chain to it
Step 7: DONE!
to customise it, you can add duct tape to give it a chrome-like finish or add spikes!