Found a pic of my little brother posing on my M20 at our local rifle range, you can just see a rifle slip behind in him in the sidecar box. I recall I had a large paving slab in the bottom of the box to counteract the lack of sidecar body weight and to help keep the wheel near the ground on left hand bends!
Kev, I've a copy of similar picture of one of my brothers (next one up from me) sitting on My Dads BSA M20/M21, seating arrangements were My Dad and oldest brother pillion on the bike, my Mum in the side car with the other two.......