The problem with filling a bag with plastic is that an arrow will generate heat from friction as it penetrates. It will melt the plastic to the shaft and it's a bitch to scrape off. Instead, go to Goodwill and buy a box of rags. Those will stop an arrow nicely without melting.

“In a time of deceit telling the truth is a revolutionary act.”
― George Orwell

It's not over when you lose. It's over when you quit.