Hello - Just joining the forum as I await my RR Step through in October!!  Yikes.    Regarding seats, I have used this one for about 15 years now, with one replacement for wear during that time.  I will see if it fits the Rad when it arrives and if not I might have to go with one of the other suggestions in the thread.  I will say that for me this seat eliminated all the saddle sore experiences I have had with any previous seat.   

https://amzn.to/3iqU0A3

Cheers - Maisie