Hi @skeggley, you can always make one yourself, that way you would be sure it would fit. I gave the link to step by step instructions from Rusty Burlew in this post:
The concept is that you make a very shallow box with canvas across the bottom of the box. Some people put vent holes in the side of the box. You fill the box with absorbent insulating material, like wood shavings, then any winter condensation gets absorbed and doesn’t drip back down onto the bees. Very simple - you just put the box on top of the hive, under the roof and leave it there all winter.