This may not be simple...
First, how will you transfer Rick to the sidecar? Maybe a Hoyer lift if there's one handy, or at least a couple strong folks. Second, from the looks of his seating system Rick has pretty serious stability and positioning challenges. I'd try to copy the positioning setup on his wheelchair, and maybe various foams would be best to work with as they're fairly easy to cut and manipulate to the desired shapes. We have any OTs, PTs, orthotists, etc. here that can offer some advice? And of course the foremost expert to consult is Rick- he knows best what works for him.