Sorry I missed your message for so long.
I bought 2 1 1/4 U bolts at Home Depot. They come with a cross bar on the bottom where the nuts are screwed in. I bought 2 so I would have 2 cross bars. I put one at the top and one at the bottom. Home Depot also sells flat pieces of rubber. I cut strips to go between the cross bars and the trike boom so they would not scratch it.
It has worked great. I haven't had to adjust the seat since the U bolt was installled. Let me know if you have any other questions.