Hi people,
I know this is an older thread but I thought I would share what I found was used on the Rad Rover in the pictures above. I liked it so much myself, I thought I'd do some research and found it. They used a mountain bike shock under the seat through a pin and probably attached it to the luggage rack below although it is hard to tell from the 2 pics. If you do a search on ebay for a MTB shock absorber, "100MM 1500LBS Rear Suspension Spring Shocker Absorber Bicycle Mountain Bike" is the heading for the one I purchased, you will see it is exactly the same spring mechanism as what is pictured on the Rad Rover. They look way cool attached to the banana seat
Hope this helps someone out.
Em.
Just to add, there are numerous sellers that sell these and they are available in different lengths. It just so happened that I needed a 100mm one.