At the risk of sounding like a cracked record (nobody under 50 will know what I'm talking about!) and/ or someone in their pay, let me suggest and recommend the BAGSTER seats from France. I await my just-ordered GT seats from them, and this will be my third or fourth such purchase for a Tracer. True - they are not 'custom' in the sense that one can visit the factory/ workshop and be measured and assessed and fitted on-the-spot by the seat-builder, which is what I believe some after-market seat-makers offer. Clearly, this might be the ultimate made-to-measure offer.
But BAGSTER seats are beautifully and flawlessy made, and can be fully customisable in terms of your preference for any of a wide range of elements, including the all-important seat foams, as well as the covering materials in over 70 colours and types of finish; colour and type of stitching; decorative embroidery; height; gel and heated options, etc. They are not cheap - landed here in Oz, about AUD$450 to $550 depending. But worth every cent IMHO.