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.