You may want to consider a
Surly Dingle cog setup with dual chainrings so you can have two easily switchable gear ratios.
"Dingle Cogs are part of a different concept for fixed-gear drivetrains. Having two cogs on the back means you have more options for gear changes when the conditions demand it. For instance, say you want to ride your off-road fixie from your house to the trailhead, but your gear combo is either too high for the dirt or too low for the road. With a 17/19t Dingle on the back, pick two chainrings that are 2 teeth apart, like a 44t and a 42t. When you change from the outer (44:17t) gear combo to the inner (42:19t), you’ll have a much better off-road gear and your wheel position will not change. This maintains effective chainstay length so you won’t have to worry about having too much or too little chain length to accommodate the gear change. The Dingle (the word derives from from dual and single) works great in hilly terrain both on or off-road. Like our cassette and track cogs, these are made from machined, heattreated and chrome plated SCM415 CroMoly steel, so they are tough and long lasting. They’re available in 3/32" (narrow chain) tooth width 17/19t, 17/20t or 17/21t combinations. We recommend using 9-speed chains only. You can thread it onto any standard ISO threaded (1.375x24tpi) fixed gear hub and it takes up the same 7.4mm threads as a standard fixie cog. We recommend using a wider lockring (like our improved track lockring) to assure you can get a lockring tool onto it."
That plus a
Surly Travelers Check [mentioned above] seem like it meets your requirements quite well while still being off the shelf.
