Ad: This forum contains affiliate links to products on Amazon and eBay. More information in Terms and rules
If I decided to have the contrast of a leather rim, which I probably would, then I'd apply a thin roll of 'Milliput' to the resin seat rim, and trim and smooth it to shape, painting it when set.
Other types of putty could be used of course, but the 'Milliput' 2-part putty is ideally suited to this type of work, and easy to use, apply and mould.