I have a Indian Roadmaster/Roadsmith trike. Great trike! Unfortunately Indian and Roadsmith do not offer rear passenger handles.
To remedy this I purchased 4 Black cut Motorcycle Rear Seat Bolt Tab Screw Mount Knob Cover Nut For Harley US on Ebay
I cut the original seat strap in half, measured, measured and measured again. Drilled holes in the fiberglass body and mounted my creation. I used a 8mm bolt with a stainless steel lock nut, fender washer and a rubber washer underneath.
My wife LOVES it!