I took a touring one up seat and adapted it to fit my Classic. Not difficult, but not for the faint of heart either. The standard latch has to be turned 180 degrees, so to use the same cable my key is now on the shifter side, or shall I say, a , ahem, the correct side. A new hole is put in the side cover, a block off plate fills the old hole, an aluminum angle is riveted to the seat pan and is used to mount the latch stud. The plastic seat pan was heated with a hot air gun and the "wings" opened slightly so that they fit over the upper shock bolts. Then I made a laminated hardwood stand to hold a back rest pad from a Triumph sissy bar. The hardwood stand is necked down so that it is narrow and thus becomes a spring which lets the back rest float a little rather than being rigid. The back rest is mounted via a swivel bracket that lets the back pad rotate forward and down or up and back. This feature along with the spring stem makes this the most comfortable backrest I have ever leaned against. It always "fits". The stem fits the square seat socket and is bolted from underneath with four small lag bolts. Sanded, primed and painted black. Numerous other little details but nothing that can't be easily done with a little time and skill.
The seat has had the cover removed and the foam contoured to add "cheek" reliefs. I can't tolerate pressure on my tail bone so this was essential to me as the stock Triumph seats are rounded up as they go back so they put pressure forward on my backside which drove me crazy.
It turned out well. If you don't know Triumph Rockets and what came on which model you wouldn't even notice that it is not stock.