Side car crib / co-sleeping

Hi I have an 8 week old boy who currently sleeps in our bed and we are wanting to sidecar a cot to the bed just for a bit of extra room but we are a bit unsure of what would be OK with the height etc

The cot is slightly lower than the mattress on the bed and I have read this is not safe however when I have looked online I have seen several pictures of side car cots that are lower than the main bed mattress (I'll put an example in comments)

Can someone tell me what is safe/isn't?

Also I have previously seen people use pool noodles the bridge the gap/fill in the side of the cot where the mattress has had to come over slightly to meet the main bed mattress? Is this a good option too?

Thanks 😀