I would look to build a frame with dropouts similar to to the All City Super Professional or the Otso Warakin. Paragon Machine Works makes a lovely version as well if you are going custom. The only place I would do an eccentric would be on a tandem or I absolutely had to turn whatever random frame I had into a single speed that I wasn't changing gearing on or anything but I wouldn't do that personally. I would get the frame modified to be a single speed/fixed gear or better yet just get a dedicated frame with the geometry I wanted or find one of the available options like the AC or some of the Otso frames (they have others with similar dropouts) and potentially others. I hate the look of the conventional sliding dropouts for QR or thru axle but the enclosed stuff looks good or if I am building a plain old road fixed gear I would just use track ends and not bother with anything else.