Baby not sleeping in bassinet...

...is there anything I can buy to put in his bassinet that makes him not lie flat? As of right now he will only sleep with us in bed, and RARELY in his swing (pediatrician gave us the ok) while its swinging. I don't think he likes being flat on his back.

We dont have the money to buy anything expensive, but I was wondering if there was anything out there for a solution besides buying a co sleeping bassinet as they are too expensive atm.