From the CRL for Modern Kits
Back covering:
Some form of back covering is required. (Backpack, back armor, air tank, cape, cloak, etc.)
Must cover at least 2/3 the overall width and length of the back.
Back armor SHOULD be used as all canon references for Modern Era Mandalorians show back armor. However, if your Mandalorian wears a duster or other jacket or a cape that fully covers the back from shoulder to shoulder and neck to waist, the use of a backplate is not required. Backpacks and jetpacks are also acceptable in lieu of a back plate. However, the back covering must completely cover the back as to make it unknown that there is no backplate present.
Soft back coverings can be substituted for back armor pieces as long as they add to the overall look of the applicant's armor.
Full/half capes are acceptable as long as they cover the entire back area. (No armor vest should be showing.)
Soft/hard backpacks/packs are acceptable alternatives.