M B hives are top bee space and the Q excluder they supply is a soft floppy plastic thing. The best solution is to frame the excluder on the top face ensuring that a batten is included at right angles to the slots to support the centre of the beast and prevent sagging. I used to use M B hives but quickly changed over to Swienty poly hives as these are completely interchangeably with my wooden national bits and don't cause me with compatibility problems.