I use leather straps over a suitcase ( non MMC one) . I have the cobra's head high level brake light and the suitcase obscures it. I modified the power supply to it to enable an LED brake light to be fitted ( simple Superseal connectors mod). This light is secured to the suitcase by a simple ratchet strap horizontally wound. Simple swop of connection from one to the other and Superseals are waterproof. This then also adds to preventing sideways slipping of the suitcase. Rock solid every trip so far. Side mirrors used as screen mirror view gets useless with the hood up.
Previously I used a Bootbag that sat lower on the rack so rear visibility good. It has its own straps. Mrs sospan wanted a suitcase so it is our reserve now.
The cheap but effective ratchet straps ( about 25mm wide) are great. I use them to strap my golf trolley to the rack. Clubs in passenger well/seat. A bad round? The drive home compensates!

Plus Four MY23 Furka Rouge