I used a Surly Singleator for about 6 months, and for those first six months it was great.
Then the spring bent.
On the last lap of the Thetford Summer Series.
I fiddled around with it trying to convince myself that I could make it work somehow, but I finally gave in to the inevitable and bought myself a new frame with proper drop-outs (On-One Inbred).
Don't buy a tensioner.
They are marketed to suck you in to the dark, addictive world of single-speeding. It seems like such a simple change at first; what harm can it do just to lose a few gears? It's only later, when you find yourself craving steep hills, or fast twisty single track, that you realise there's no going back, that you're hooked, with no possibility of cure or remission.
Your friends, who at first were so admiring of your "cool" single speed bike, now shun you, afraid of what you have become. You no longer to talk to your family, except to mutter about "chainline" or "drop outs". Your stunning FS sits disconsolately in a corner of the garage gathering dust, abandoned and forgotten, loved only by mice.
Just say no.