Many thanks for your reply, Scott. Very informative!
I’m afraid we have very few, if any, such mom-n-pop shops over here in Sweden. The upholstery shops we do have are very expensive (my last bill was $6500 for new vinyl on my bench and two seats). Asking them to build a new seat would be ridiculously expensive.
I had hoped to just be able to order a wide helm seat somewhere online but your post has really got me thinking about building one myself instead. The TACO website that you referred to shows the backrest support hardware I’d need (
https://tacomarine.com/taco-seating-hardware-m20-0035 ) but ordering a pair seems to be difficult. They’re just nowhere to be found online! (I’ve only found a single suspicious-looking web site selling them but the web site has gotten several scam warnings so I will stay away from them). I’ve also written directly to TACO but they only accept $1000+ orders.
But I’ll keep looking, I know what I need now. Hopefully I’ll come across a pair somewhere so that I can start building the seat as a winter project.