The Axiom Fatliner Fat Bike Rear Rack happens to work perfectly with the Otso Arctodus with the rack's stock hardware.

Product Info from Axiom's website:

Compatibility: 26” FAT (up to 5.0”)
Stay length: 427mm / 16.8”
Platform size: 355mmx170mm / 14”x6.7”
Weight: 982g / 2.16 lbs (including hardware)
Capacity: 50 kg / 110 lbs

Part Number: 171268-01 - Black