Knowing nowt about upholstering a bike saddle I'll chuck in my extra penny on saying take note of which glue you intend on using.
My limited knowledge of upholstering is from furniture upholstering and its all about pulling the fabric taut and even and then staple/pinning the edge to hold the shape.
Contact glues might not allow any repositioning, which would make it difficult to get evenly tight :?. maybe look to something with a less permanent bond, and its the pulling and smoothing the fabric around the contours before pinning it that holds the shape.