Over the years folks have come up with ways to make rebar safe: tennis balls, plastic bottles, candy cane bends.
Times have changed.
Lowes and Home Depot (and any construction supply) now stock rebar caps in bulk. You can get a bag of 20 or so for a few bucks. They come in a few sizes. You need the small size for #3/#4 rebar.
They are bright orange and fit neatly on the top of the rebar. By far the best answer. Very cost effective. No need to worry about other otions any more.
Cheers

