Go for H&B
 

I used their racks for 30.000 kms in Africa and another 5.000 in Turkey, without any problem. Together with Dearr boxes. Cheap compared to other options, when you order th racks from H&B or Daerr they ask your bike's VIN number so they are sure they send you fitting racks.

The lock alone is not suitable to keep the box attached to the bike. Use for that a ratchet strap and fix tightly. Then, you can still take of the boxes if you want.
Indeed, put some adhesive plastic on the inside, otherwise the contents will be covered in grey dust.

Auke