I did it once, long ago. Actually, I was customizing a hard plastic bodied
saddle. I drilled a few holes in the places my sit-bones hit to give it some
more flex, and built up cushioning lumps using RTV silicon. Then I stretched and
glued a new leather cover over the saddle. IIRC, I used contact cement.

It wasn't difficult, and it worked very well. The saddle's still good, although
I haven't been using it for quite a while.

- Frank Krygowski