I dont know about knock-off, but I've used this Amazon mattress for over a year (300+ sleeps) and I love it and it doesn't hurt my back or anything. Highly highly recommend.

I started with one and it was great. Eventually added a second that I cut to fill out the rest of the space. If I have a passenger or need the extra space i'll throw the cut up mattress in the trunk and leave the single.

https://imgur.com/gallery/sxAu4Dg

Here's my setup with two

Olee Sleep Tri-Folding Memory... https://www.amazon.com/dp/B0742B6F1H?ref=ppx_pop_mob_ap_share