Sure, just let gravity do it’s thing. Just hope you don’t trip and all the bits fall out. 
Then gravity is his enemy. There was twine on the blanket as a backup. Do you think he used that?
Even if we agreed that it would have been better for Oswald to have sealed both ends of the bag (and I don't agree) so what? The fact that he may not have performed every task associated with the assassination with optimal efficiency does nothing whatsoever to negate the actual evidence that links him to the bag and crime. It's pretty weak sauce. Assassinating the president is fraught with enormous risk. I doubt Oswald ranked the risk of his bag coming open in the presence of Goober Pyle (Buell Frazier) or tripping very high that day in comparison. And if it happened, he would spin some Ozzie tale.
