I have exactly the same problem. I called bednest to ensure I could return and then just went for it. It seems to work really well. Havent tested with baby yet as he is still inside my belly! There is a fold down side on the bednest that works as a bridge between the beds. I have a thick mattress topper that I have slid over a cm or so to make the fit even snugger. If I were you I would buy it and try. You can always return it?
I phoned Bednest and sent them measurements and photos of the Malm and they said it should work. I ordered it and received but wasn't happy at all when I tried it out.
There was a gap so what's the point? I felt it would be quite unsafe with the side down. The gap was big enough for a limb to get wedged in and anything like a bolster could be a suffocation risk. The side has some ridging boning in it too which I thought might make baby's transfer along bridge uncomfortable.
They were good about me returning it.
The Babybay should work if you buy the extension kit. It's bigger too.
I went with the Fabimax which I ordered from Amazon France. You can search for it under 'lit cododo'. It came with extension kit so joins up to my mattress perfectly.
The only thing is when you have the extension kits on either Babybay or Fabimax you can't have the side on. That would mean you shouldn't leave baby in there unsupervised if you thought they might be able to get out on to your bed.
I've also heard quite a few people stuffing the gap with rolled up towels or a folded blanket then folding the soft side of the bednest down over that. I think you probably need to suck it and see - order it, see what you think and just make sure in advance that you can return it if it doesn't work.