A bit like trying to get into a pair of jeans which is one size to small – kind of fits but not comfortable and some pressure….
It is EXACTLY that 😅
I use the EZ-roller that’s made for the Mini 4 & 5. Took it to the Apple store and slid a Mini6 into it. As the 6 is a touch shorter than the 5, the top roller has some play, but I don’t think that’s a big issue. Of course haven’t tried it in the airplane. My guess is that it’s fine if positioned vertically, perhaps not so much if mounted horizontally.