You DO need proof of onward travel when applying for a SETV. But I there is a trick almost everyone does.... Book on Expedia (24 hour free/no fee cancelation. Book your tickets. Print out the required info. Cancel ticket. Everyone who does border bounces does this.
buy a tix that will let you cancel free within 24 hours (expedia) print what you need. Cancel the ticket. It's what EVERYONE does when going on border bounces