If this isn't in game already:
Small, compact bike seats that are placed in line with the cross section of a construct; a block that covers the form of a person seated as they would on a motorcycle. I wouldn't just add a cushion and call it a job well done, as then the players legs could potentially clip through any other parts.
The outline in red is what chunk the part encompasses
Mostly just for looks. But it could open up avenues to make lighter and more streamlined vehicles.
I see some potential form issues due to the shape of the legs. It may be hard to sculpt the form of the vehicle around it.