Dont do it! Get a stock R3T dual seat and cut it down if you must, as they are cheaper and easier to find from folks who have upgraded previously.
The backrest alone recently sold in the US eBay for US$500+ as they fit ALL Triumphs with that backrest system - Thunderbirds, Classics, Roadsters and R3Ts,
The seat is the best dual factory seat for the Classic, Std and Roadster with or without the backrest fitted and is worth NZ300-400 by itself
I have bought and sold heaps worldwide for years.
I have sold all my R3T seats now 3x duals and 4 Solo buckets with and without the backrests and seperate pillions but am not acquiring anymore as I'm downsizing and moving still.