Recover that old, ratty mountain bike seat, including a *Trendy* slot in the seat.

Step 1: Padding

Start with a ratty old bike seat.
Cut out some closed cell foam to fit the contour of the seat. (I used a scrap from some shop-floor padding, camping pad would be better.) Don't worry about getting it perfect, you will trim it later.
Cut out a slot in the top of the foam. Your *Special* artery will thank you!
Use a dab of superglue to stick the front and rear of the padding to the old seat.

Step 2: Cut Some Covering

I chose to cover the foam with a scrap of suade from another (unfinished) project. You could use just about any kind of tough cloth, leather will have a little stretch to conform to the shape of the saddle.
Sketch out a line around the saddle, leaving about an inch of extra.

Step 3: Punch Holes

Punch some holes around the edge of the suade. I used one of those leather punch pliers that punch several sizes. It broke, so the Swiss Army Knife came to the rescue... as usual.

The nail is just there to help push the lacing through the holes.

Step 4: Lace It Up

You could use anything to lace up the covering. I used 1/4" wide strips cut from an inner tube. Pull them through the holes and tie the ends together. Start from the middle and work toward both ends.
Remember to leave access for a seatbag as you lace it up. Mine attaches to that plastic bracket on the seat rails.

Step 5: Finish It Up.

Once it was mounted on the bike, I soaked the suade with some water. I'm not sure that was necessary, but I figured it would stretch out some, then shrink tigher than before. It didn't hurt anything anyway.