E*13's EX cog addsa40 or 42T crawler gear to the top end of your ratios if you remove the 17 or 15T sprocket from your existing 10-speed cassette.
Different 'timed' versions are available for SRAM or Shimano blocks and an extra long rear derailleur set screw is included with the ring, which is a nice touch. The lurch up onto the 42T cog still needs a bit of care so select it early if you can and be prepared to be patient for it to drop back down compared to a conventional cassette. It's worn fine through winter though, is comparably priced and there are blue, red and black options.
PRICE 82$
