What type of brake mount do you use?
If your unicycle is 2012 it has a mount on the frame, otherwise the best way is to get a D-brake adaptor from nimbus.
If you are talking about the caliper adapter you need one relevant to the size of the rotor you are using and make sure you get a front one as the rear ones are a size smaller as the frame mount points on a bike are closer to the rear axle drop out
mopst brekes are post mount so you need a post mount to IS (international standard) adapter
Thes come in various sizes
f160mm (same as rear 140mm)
f180mm (same as rear 160mm)
f183mm (same as rear 163mm)
f200mm (same as rear 180mm)
f203mm (same as rear 183mm)
Caliper mount is one thing. Use front one as feisty wrote.
And instead of d’brake you could use UCM (http://mountainuni.com/collections/ucm) that was planned to work with outbound disk.
The D-brake works fine on an exteranl brake as by the very nature of the rotor placement it is an ambidextrous part (the rotor is on teh othe side and so is the caliper so the caliper mounts to the d-brake on the same side). I have a crank disc on my Triton 36" and a Orcal Hub on my KH29 and use a D-brake on both
The UCM is more expensive I believe but gives a more upright caliper position which looks a little neater in my book but I don’t like silver components so went for a green and black set of D-brakes for my Unis (I am such a tart)
If you are converting an older (pre 20012) KH frame which hasn’t got the welded on caliper mounts and have to use a D-brake UCM mount you may find clearance issues between teh rotor bolt heads and the frame bearing holder / leg weld, a quick file or preferably using counter sunk 10mm rotor bolts will solve the problem.