Did the same thing for my 84 Rider. Picked up a brand new seat from a guy on Kijiji who had a few to sell from overstock at a marina. I think I paid about 200 for mine IIRC. I like the bolster for when I'm driving through shallow water, it makes it easier to see over the side. My seat looks very much like yours, and like you said, it was a very tight fit. I'd say squished right up against the side, it still might be about an inch off centre. The main reason I got it though, was we couldn't get the old seat set up nicely for my wife.