The  new Quick release BUMOT Defender EVO top cases  43L or 30L   is a perfect solution to get needed capacity . 
Defender EVO top cases require a BUMOT top case mount. Includes BUMOT's new bespoke latches in stainless steel, a reinforced case floor in the points of attachmen, that provides a 4mm mounting surface for increased strength, and lower profile lid strap brackets in high strength polyamide nylon for increased pillion comfort and less likelihood of injury in an "off". 
Starting wtih 2mm aluminum, BUMOT's unique assembly process employs a technique used in the sports car and airline industry to double the bottom edges and floor of the box from 2mm to 4mm.  BUMOT top cases are powdercoated inside and out in your choice of raw aluminum , frozen grey or black, and are finished with zink passivated  friction hinges, stainless steel latches with keyed locks, and heavy duty weatherstripping for waterproofnes

Dimensions :

Top case 43L- 41/33/34(H) cm

Top case 30L - 41/33/24.5(H)cm