i recommend reading up on the White Industries ENO Eccentric hub.

if your frame will accept a 126mm, or 130mm, or 135mm (disk only) OLD rear hub. you won't need to worry about your vertical dropouts, bottom bracket, or using a chain tensioner. look up OLD if you don't know what that term means WRT bicycle frame and wheel dimensions.

it's a flip-flop hub offered in 28, 32 or 36h drillings and supports fixed cog or single-speed freewheel.