Here's how to make a tandem seat for a moped or motorcycle from any cushion that's roughly the right size and shape.
I got this armrest cushion from a boatyard dumpster.
I suspect the fabric is UV-resistant.
Step 1: Before
You can do it, but it's uncomfortable and probably dangerous.
Remove the seat. Ride around on the moped like that.
Believe it or not it's already greatly improved!
Step 2: Cut Off the Protruding Seat Support
Cut it off So it's about level with the remaining luggage rack.
Smooth off the remaining jagged burrs.
Step 3: Wooden Seat Base
This one is 8"x27".
It's 3/4" poplar pretending to be birch.
Round the corners.
Round the edges.
That will make it less likely to bite holes in the cushion or you.
I used a tablesaw, beltsander, and router table with 1/2" diameter qarter-round bit, but a handsaw, knife, and sandpaper would work just as well.
If you can't find the right kind of cushion, it's easy to modify one if you have a sewing machine,
or you can just upholster your board as seen in other instructables.
Step 4: Mark and Drill Boltholes
Whittle a skinny pencil to reach through existing holes in the luggage rack and mark your board.
Drill holes for the bolts.
I used 1/4" -20 thread per inch carriage bolts.
Step 5: Add Bolts
These bolts are good because the wide head has a square nub to keep the bolt from turning.
Perfect for carriages in fact.
Step 6: Put the Board Inside the Cushion
I unzipped it.
pulled the foam block out.
Put the board in with the protruding bolts facing down.
poked the protruding bolts through the fabric.
Stuffed the foam block back in
zipped the cushion back up.
Step 7: Bolt and Enjoy!
Then I took a ride on my cushy new seat.