On the topic of singlespeed tensioning solutions:
I recommend running a single half link in a compatible chain instead of a half link chain (KMC Z1 and HL1 work well together, I tested the 1/8" versions, but I bet the 3/32" variants would work too). Way cheaper and you have half-links for life, plus any durability concerns people regularly voice about half link chains no longer apply to you :)
For magic gear ratios, there's this here gem of early 2000s webdesign: https://eehouse.org/fixin/fixmeup
I can also second that a BB mounted chain roller works nicely as a fixed position tensioner (I used a Blackspire Stinger, clamped by the BB, with no issues).
Similar fixed wheel tensioners exist to be mounted instead of a derailleur (DMR make one if I remember correctly).
Good sprung tensioners are available from SB-One (mounts on the derailleur hanger, clears only a 17t, but is indeed a nifty piece of kit), and I think I saw someone use a pinion tensioner meant to be mounted on the Gearbox hacked to some ISCG tabs. I've heard mixed things about the Rohloff and Alfine tensioners. When I last tried to source a Yess tensioner I couldn't find one over here in europe, sadly.
Finally, there's eccentric BB conversions for BSA frames at least (I can only think of the Trickstuff Exzentriker), but that's expensive, doesn't give much tensioner range, and restricts you to run 24mm cranks (though I personally don't see an issue in that).
As a curiosity, I'll also add that Ghost Rings are a thing: http://rollinginboston.bostonbiker.org/2012/04/08/installing-a-ghostring-on-a-single-speed-or-internally-geared-bicycle/
Not sure I'd want that on an MTB though.