DIY Stern Seat

I am looking to add a stern seat to my raft for fishing. I currently have a very simple 3 bay frame. Any suggestions on how to add a stern seat? I am hoping to do it cheaply, so building it myself seems like the way to go.

My current idea is to add on a back bay and use U bolts to mount a seat. Will U Bolts attached to a cross frame keep a chair from twisting?