We have done a ton of bike seats in my shop. I do embroidery on leather products but I've seen our upholsterer do tons of these. He cuts the seat foam with an electric carving knife (like you'd use for a turkey) just deep enough to flush up the gel pad. Glue it in place. Then you wrap over the entire seat with 1/2 inch soft foam (gluing again) and upholster. Voila! Finished gel seat. We usually secure the gel inserts for $50 and charge $125 to install them up and above the cost of the upholstery. Easiest $75 ever.
