I thought I'd post up my diy gel seat.. Its not strictly on a sv altho my first attempt was to make a lowered seat.
Ingredients
1) seat
2) gel pad
3) screw driver
4) dremel type thing
5) spray adhesive in a can ( make sure its ok for vinyl i think)
6) face mask and goggles
7) staple gun

hammer (optional)
9) time
This is only a guide and I accept no liability for anything that is caused as a result of the following text and pictures.
Take the staples out using a flat head screwdriver. You only need to take 3/4 way round. Basically just enough to lift the cover off.
Cut the fabric from the gel seat cover and draw round for a template
Dremel out the space. Make it smaller at first. You can always remove more later.
Check the fit by placing in hole. Then use the spray adhesive and coat both sides and place in hole. Stick a box on top whilst its drying.
Spray more adhesive on top and slowly press down the cover on top of the seat. Do half at a time and reapply the staples. The holes wont line up so new staples will be required. Do the rest and allow to dry.
Reattach to bike and test ride.
Hope its of some use to someone.