Child bike trailer extra padding

I have just bought a second had trek rocket which I believe is similar to the chariot cheetah, my son is only 6 months old so I’m looking at putting some extra padding in the seat to support him. Does anyone know if something like this http://www.thule.com/en-us/us/products/active-with-kids/multifunctional-child-carriers/accessories/thule-baby-supporter-_-20101001 will fit? Or if there are any universal baby support kits that would do the same thing?

Cheers