Hi SaraJane & Marlene,
Look at me now thinking I'm an expert on bedrails! LOL. I have a slat bed, and the two 'legs' of the bed rail position nicely over a slat each.
My mattress sits about an inch 'into' my bed frame and the rail 'feet' just sit nice and snug against the bed frame. If you aren't able to push your mattress a tiny bit over then it might not work for you.
I found that all the other bed frames are made for small mattresses and because we have a pillow top, the top of the frame was about 1cm higher than Seven, so kind of pointless!
Also I didn't find them very stable.

I can take some pictures if you like?
Also, I have a couple of suggestions for cosleeping aids if you want.
Hope this helps!