Gap down the side of mattress what should I fill it with?

[Mummyhug]

DD is now 7 months and has been sleeping in the next2me so far but has now grown too long for it. I'm not ready to move her to her own room yet and I love having her by my side in the next2me so I decided to put up the cot bed in my room and have it at my bedside with no side on just like the next2me.
It's perfectly level with our mattress which is a relief but even with it pushed right up to the bed there's a gap between the mattresses because of the frames of my bed and her cot. About 3-4 inches maybe.
I can push her mattress right up to mine and that works fine except then the gap is now at the other side by the bars.
I've temporarily stuffed the gap with a duvet to stop the mattress sliding about and losing DD down the gap but I'd like to do something more permanent.
Any ideas?

We had that problem and used a tightly rolled up beach towel.

Pool noodle

Or foam bed guarf

I don't think a towel would cut it, I've got a full single duvet down there at the minute although it does bulge out quite a bit. I've wrapped hair bobbles around it to keep it from unraveling, looks like a caterpillar
I did think of a pool noodle, I think it should be about the right size

Just googled a foam bed guard, never heard of them before but they look fantastic! Might be good for my four year old too actually