I ran into the same issue, I cancelled my subscription in May 2015 -- it was thru iTunes. I had an existing HBO Now profile. About a week ago I purchased the new 2016 Roku Streaming Stick and tried with that, it worked and I got a free 1-month trial. The sign-up process generates an email with a link that once I clicked on, it swapped out the previously cancelled subscription in my profile with the new Roku one which allows me now to use the ATV4 HBO Now app with my pre-existing login. The ATV4 HBO app is much nicer than the Roku one.
Showtime didn't fall for the same trick though, prompted me to renew. Another option is SlingTV, they offer a 7-day free trial and the HBO Free Preview Weekend just works without signing up for HBO.
Sort of like getting a $15 discount on the streaming stick. MHz Choice also has a free month trial, another $8 discount. I mainly got the Roku for the Amazon Prime Video app, but am pleased with these discounts