Plenty of aftermarket sissy bar/racks for much less than Triumph asks, and you can always add a back rest to a luggage type rack.
Cheap ebay backpads look reasonable for price and they have many variants. Copy below from an earlier post #27 of mine here
Utopia backrest too big??
The links may take you too an old listing but pretty sure you will find they have been relisted or the seller/s have more.
AUD$30 delivered looks quite good
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AUD$10 bucks delivered to try concept is pretty cheap and no cutting your expensive one.
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$20 delivered studded 3 threaded holes with measurements-
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The 'sofa' look
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tall 'sofa'
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'Triangulate' version
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Heaps more from this site
backrest pad | eBay