Omnium Single Speed 15T Converter Kit

Here's what Omnium has to say:

Single-Speed adaptor kit for 8/9/10/11 speed Shimano compatible cassette hubs.

Pair of CNC machined aluminium spacers offer lateral support to a ‘deep-tooth’ single sprocket.

Built-in offset optimises chainline.

Supplied with 15T 3/32″ cog and lockring.

NOTE: Cog teeth are 3/32″ – 6/7/8 speed or track chain compatible.

Not suitable for narrow 5/64″ (9 speed) or (10 speed) chains.